Showing papers by "University of Crete published in 1996"

Unified nomenclature for broadly conserved hrp genes of phytopathogenic bacteria

Abstract: Genes of plant-pathogenic bacteria controlling hypersensitive response (HR) elicitation and pathogenesis were designated ‘hrp’ by Lindgren et al. in 1986 (J Bacteriol 168: 512–522). hrp genes have been characterized in several species of the four major genera of Gramnegative plant pathogens, Erwinia, Pseudomonas, Ralstonia (a new proposed genus including Pseudomonas solanacearum) and Xanthomonas. To date, hrp genes have been found mainly in large clusters, and they have been shown to be conserved physically and, in many cases, functionally among different bacteria. Hybridization studies and genetic analyses have revealed the presence of functional hrp genes even in species that are not typically observed to elicit an HR, such as Erwinia chrysanthemi and Erwinia stewartii, suggesting that hrp genes may be common to all Gram-negative plant pathogens, possibly excluding Agrobacterium spp. Current knowledge of hrp genes has been reviewed by Bonas (1994, Curr Top Microbiol Immunol 192: 79–98) and by Van Gijsegem et al. (1995, In Pathogenesis and Host–Parasite Specificity in Plant Diseases: Histopathological, Biochemical, Genetic and Molecular Basis. Volume 1. (Kohmoto et al., eds); Oxford: Pergamon Press, pp. 273–292). The nucleotide sequences of four hrp gene clusters, those of Ralstonia solanacearum (previously P. solanacearum) (Genin et al., 1992, Mol Microbiol 6: 3065–3076; Gough et al., 1992, Mol Plant–Microbe Interact 5: 384–389; Gough et al., 1993, Mol Gen Genet 239: 378–392; Van Gijsegem et al., 1995, Mol Microbiol 15: 1095–1114), Erwinia amylovora (Bogdanove et al., 1996, J Bacteriol 178: 1720– 1730; Wei and Beer, 1993, J Bacteriol 175: 7958–7967; Wei and Beer, 1995, J Bacteriol 177: 6201–6210; Wei et al., 1992, Science 257: 85–88; S. V. Beer, unpublished), Pseudomonas syringae pv. syringae (Huang et al., 1992, J Bacteriol 174: 6878–6885; Huang et al., 1993, Mol Plant–Microbe Interact 6: 515–520; Huang et al., 1995, Mol Plant–Microbe Interact 8: 733–746; Lidell and Hutcheson, 1994, Mol Plant–Microbe Interact 7: 488–497; Preston et al., 1995, Mol Plant–Microbe Interact 8: 717–732; Xiao et al., 1994, J Bacteriol 176: 1025–1036), and Xanthomonas campestris pv. vesicatoria (Fenselau et al., 1992, Mol Plant–Microbe Interact 5: 390–396; Fenselau and Bonas, 1995, Mol Plant–Microbe Interact 8: 845–854; U. Bonas, unpublished), have been largely determined. These clusters each contain more than twenty genes, many of which encode components of a novel proteinsecretion pathway designated ‘type III’. It has been shown directly that various extracellular proteins involved in pathogenesis and defence elicitation by plantpathogenic bacteria utilize this pathway (Arlat et al., 1994, EMBO J 13: 543–553; He et al., 1993, Cell 73: 1255–1266; Wei and Beer, 1993, ibid.), and the pathway is known to function in the export of virulence factors from the animal pathogens Salmonella typhimurium, Shigella flexneri, and Yersinia entercolitica, Yersinia pestis, and Yersinia pseudotuberculosis (for reviews, see Salmond and Reeves, 1993, Trends Biochem Sci 18: 7–12; and Van Gijsegem et al., 1993, Trends Microbiol 1: 175– 180). Nine type III secretion genes are conserved among all four of the plant pathogens listed above and among the animal pathogens. Based on sequence analysis and some experimental evidence, they are believed to encode one outer-membrane protein, one outer-membrane-associated lipoprotein, five inner-membrane proteins, and two cytoplasmic proteins, one of which is a putative ATPase. All of the predicted gene products, except the outer-membrane protein, show significant similarity to components of the flagellar biogenesis complex (for reviews see Blair, 1995, Annu Rev Microbiol 49: 489–522; and Bischoff and Ordal, 1992, Mol Microbiol 6: 23–28). We herein refer to the hrp-encoded type III pathway as the ‘Hrp pathway’. Because hrp genes have been characterized independently in diverse plant-pathogenic bacteria, hrp gene nomenclature differs in different species, and it is not always consistent even within the same organism. Different designations are used for homologous genes, and, even worse, the same designation is used for different genes in different organisms. For example, hrpI of E. amylovora is homologous with hrpC2 of X. campestris pv. vesicatoria and hrpO of R. solanacearum, and the homologue in P. syringae pv. syringae appears in the literature both as hrpI and as hrpJ2. Also, ‘hrpN ’ in R. solanacearum designates a secretion-pathway gene, whereas in E. amylovora, ‘hrpN ’ designates the gene encoding the elicitor harpin. Furthermore, in many bacteria the number of known hrp genes approaches 26. In anticipation of exhausting the alphabet, some authors chose to designate hrp genes with a letter and a number, creating the potential for confusion of distinct genes with alleles of the same gene. For hrp gene researchers, the current nomenclature is at best inconvenient; for other scientists, it is bewildering. Another problem exists: accumulation of knowledge about the structure of hrp loci has outpaced the accumulation of Molecular Microbiology (1996) 20(3), 681–683

Wolbachia infection and cytoplasmic incompatibility in Drosophila species.

Abstract: Forty-one stocks from 30 Drosophila species were surveyed for Wolbachia infection using PCR technology. D. sechellia and two strains of D. auraria were found to be infected and were tested for the expression of cytoplasmic incompatibility, along with D. ananassae and D. melanogaster strains, which are already known to be infected. D. ananassae and D. melanogaster show levels of incompatibility up to 25%, while D. auraria and D. sechellia exhibit levels of egg mortality ~60%. A dot-blot assay using the dnaA sequence as probe was developed to assess the infection levels in individual males that were used in incompatibility crosses. A positive correlation between bacterial density and cytoplasmic incompatibility was observed. The stocks examined can be clustered into at least two groups, depending on the levels of infection relative to the degree of cytoplasmic incompatibility exhibited. One group, containing D. simuluns Hawaii, D. sechellia , and D. auraria , exhibits high levels of cytoplasmic incompatibility relative to levels of infection; all the other species and D. simulans Riverside exhibit significantly lower levels of cytoplasmic incompatibility relative to levels of infection. These data show that, in addition to bacterial density, bacterial and/or host factors also affect the expression of cytoplasmic incompatibility.

Social influences on false belief access : Specific sibling influences or general apprenticeship ?

Abstract: Some recent studies have found a relation between the number of siblings 3–4-year-old children have and their performance on false belief tasks. 2 experiments reported here examine a variety of factors in children's social environments, including daily contact with peers and adults as well as the numbers of their siblings, on a battery of false belief tests. In Experiment 1, 82 preschoolers were studied in Rethymnon, Crete, in order to obtain a range of extended kin available as a resource for the child. In Experiment 2, 75 Cypriot preschoolers were studied in Nicosia in order to examine the influences of each child's daily social contacts, as measured by maternal questionnaire. Logistic regression revealed that the factors which account for most of the predicted variance on the theory of mind tests were (a) the number of adult kin available (Experiment 1) or adults interacted with daily (Experiment 2), (b) the child's age, (c) the number of older siblings a child has, and (d) the number of older children interacted with daily. The results suggested that theory of mind is not simply passed from one sibling to another in a process of social influence. It seems more likely that a variety of knowledgeable members of her or his culture influence the apprentice theoretician of mind.

Concentration fluctuation induced dynamic heterogeneities in polymer blends

Abstract: The presence of two distinctly different local segmental mobilities found in the case of several phase mixed polymer blends by two‐dimensional 2H‐NMR, dielectric spectroscopy and depolarized dynamic light scattering is rationalized through a simple concentration fluctuation model. Our primary hypothesis is that, although the probability of the occurrence of concentration fluctuations is symmetric about the mean value in a given volume, the ‘‘cooperative volume’’ over which a fluctuation must occur for it to be detected by a dynamic probe is not a constant, but rather depends on the composition of the cooperative volume. Consequently, we suggest that the cooperative volume associated with a concentration fluctuation be determined by the local composition in a self‐consistent manner. In the case of systems with weak interactions and large Tg contrast, these ideas are shown to create a bimodal probability density function for dynamic concentration fluctuations, which has a local maximum corresponding to smal...

Tourism and the family in a rural Cretan community.

Abstract: Several works on change deal with the effects of industrialization on family life. This paper presents evidence of family change due to mass tourism in a rural community on the island of Crete, Greece. It argues that such change is attributable to economic rather than ideological factors, introduced by outsiders. The data reveal that the influence of family control, the importance of marital arrangements, and the dowry system have not lost the significance they enjoyed before tourism. However, endogamy patterns have changed and the marital age-gap has widened. While independence of working women has undergone qualitative changes, sexual freedom has not occurred equally for young males and females.

UV-B-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence. Impairment of donor and acceptor side components.

Abstract: Inhibition of photosystem II electron transport by UV-B radiation has been studied in isolated spinach photosystem II membrane particles using low-temperature EPR spectroscopy and chlorophyll fluorescence measurements. UV-B irradiation results in the rapid inhibition of oxygen evolution and the decline of variable chlorophyll fluorescence. These effects are accompanied by the loss of the multiline EPR signal arising from the S2 state of the water-oxidizing complex and the induction of Signal IIfast originating from stabilized Try-Z+. The EPR signals from the QA-Fe2+ acceptor complex, Tyr-D+, and the oxidized non-heme iron (Fe3+) are also decreased during the course of UV-B irradiation, but at a significantly slower rate than oxygen evolution and the multiline signal. The decrease of the Fe3+ signal at high g values (g = 8.06, g = 5.6) is accompanied by the induction of another EPR signal at g = 4.26 that arises most likely from the same Fe3+ ion in a modified ligand environment. UV-B irradiation also affects cytochrome b-559. The g = 2.94 EPR signal that arises from the dark- oxidized form is enhanced, whereas the light inducible g = 3.04 signal that arises from the photo-oxidizable population of cytochrome b-559 is diminished. UV-B irradiation also induces the degradation of the D1 reaction center protein. The rate of the D1 protein loss is slower than the inhibition of oxygen evolution and of the multiline signal but follows closely the loss of Signal IIslow, the QA-Fe2+ and the Fe3+ EPR signals, as well as the release of protein-bound manganese. It is concluded from the results that UV-B radiation affects photosystem II redox components at both the donor and acceptor side. The primary damage occurs at the water-oxidizing complex. Modification and/or inactivation of tyrosine-D, cytochrome b-559, and the QAFe2+ acceptor complex are subsequent events that coincide more closely with the UV-B-induced damage to the protein structure of the photosystem II reaction center.

Multiple origins of gender-associated mitochondrial dna lineages in bivalves (mollusca: bivalvia)

Abstract: Previous studies have shown that marine mussels (genus Mytilus) and a freshwater mussel (Pyganodon grandis) contain two distinct gender-associated mitotypes, which is a characteristic feature of the phenomenon of doubly uniparental inheritance (DUI) of mitochondrial DNA (mtDNA). Here we present evidence for the presence of distinct male (M) and female (F) mitotypes in three other bivalve species, the mytilid Geukensia demissa, and the unionid species P. fragilis and Fusconaia flava. Nucleotide sequences of a segment of the COI gene from the M and F mitotypes from each of the three mytilid species (M. edulis, M. trossulus, G. demissa) and three unionid species (P. grandis, P. fragilis, F. flava) were used for phylogenetic analysis. The analysis suggests three independent origins of M and F mitotypes for the six species examined; one for the three unionid species, one for the two Mytilus species, and one for Geukensia. The first of these F/M divergence events, while of uncertain age, predates the divergence of the two unionid genera and is likely older than either of the two F/M divergence events in the mytilid taxa. The most parsimonious explanation of multiple F/M divergence events is that they represent independent origins of DUI. Another possibility is that, in a given taxon, an F or M mitotype assumes the role of the opposite mitotype (by virtue of a mechanism that remains to be clarified) and subsequently was fixed within its new gender. The fixation of a mtDNA lineage derived from a mitotype of switched function would reset the divergence of the gender-associated lineages to zero, thereby mimicking a de novo split of F and M lineages from a preexisting mtDNA genome that was not gender specific. Further broad-scale taxonomic studies of the occurrence of distinct M and F mitotypes may allow for the evaluation of the latter hypothesis.

Slow Relaxation Phenomena Induced by Breathers in Nonlinear Lattices.

Abstract: We study relaxation properties of one dimensional nonlinear lattices which are initially thermalized and subsequently put intp contact with a cold bath simulated by absorbing boundary conditions. We observe a nonexponential lattice energy relaxation in contrast to the standard exponential relaxation law of the corresponding linear system. We connect the long-tail relaxation behavior with the presence of long-lived nonlinear localized modes. The mobility of the breathers is shown to play a substantial role in the lattice relaxation properties. [S0031-9007(96)01862-5]

Attenuation of multiple-scattered sound

Abstract: We study acoustic waves propagating in two-dimensional (2D) systems consisting of steel cylinders surrounded by air. We use the plane-wave method in order to get the band structure of waves propagating in an infinite periodic system. In addition, we employed, for the first time for acoustic waves, the transfer matrix method in order to find the transmission coefficient of waves propagating along a system with a finite thickness. Both methods are in good agreement with each other and their results agree with recent measurements in a similar system (R. Martinez-Sala et al., Nature, 378 (1995) 241). We also find the optimum conditions for the appearance of spectral gaps.

Speaker adaptation using combined transformation and Bayesian methods

Abstract: Adapting the parameters of a statistical speaker independent continuous-speech recognizer to the speaker and the channel can significantly improve the recognition performance and robustness of the system. In continuous mixture-density hidden Markov models the number of component densities is typically very large, and it may not be feasible to acquire a sufficient amount of adaptation data for robust maximum-likelihood estimates. To solve this problem, we have recently proposed a constrained estimation technique for Gaussian mixture densities. To improve the behavior of our adaptation scheme for large amounts of adaptation data, we combine it here with Bayesian techniques. We evaluate our algorithms on the large-vocabulary Wall Street Journal corpus for nonnative speakers of American English. The recognition error rate is approximately halved with only a small amount of adaptation data, and it approaches the speaker-independent accuracy achieved for native speakers.

Effect of nifedipine on rectoanal motility

Abstract: PURPOSE: Based on the rationale that the calcium channel blocker, nifedipine, decreases lower esophageal sphincter pressure in achalasia, a prospective controlled trial was performed to evaluate the effect of sublingual nifedipine on the anal sphincter of controls and patients with high anal resting pressures. METHODS: Ten age-matched and sex-matched controls without evidence of anal disorder and ten patients with hemorrhoids and/or fissure-in-ano were included in the study. Anorectal manometry, with an eight-channel, water-perfused catheter was performed on all patients before and 30 minutes after administration of 20 mg of sublingual nifedipine. RESULTS: Nifedipine significantly reduced anal resting pressure in both controls and patients by approximately 30 percent (P<0.001 andP<0.0001, respectively). A significant reduction was also noted in the length of high-pressure zone of the anal sphincter (P<0.02 for both groups) and in the frequency (controls,P<0.05; patients,P<0.03) and amplitude (controls,P<0.03; patients,P<0.009) of slow waves in both groups, whereas the presence, frequency, and amplitude of ultraslow waves were significantly reduced only in the patient group (P<0.05;P<0.01;P<0.0005, respectively). CONCLUSION: Nifedipine reduces the activity of the internal anal sphincter both in controls and patients with high anal resting pressure. The drug might be of some use in relieving symptoms in patients with hemorrhoids or anal fissure.

Anatomy and physiology of the primate interstitial nucleus of Cajal I. efferent projections

Abstract: 1. The efferent projections of the interstitial nucleus of Cajal (NIC) were studied in the squirrel monkey after iontophoretic injections of biocytin and Phaseolus Vulgaris leucoagglutinin into the NIC. To ensure the proper placement of the tracer, the same pipettes were used to extracellularly record the discharge pattern of NIC neurons. 2. Three projection systems of the NIC were distinguished: commissural (through the posterior commissure), descending, and ascending. 3. The posterior commissure system gave rise to dense terminal fields in the contralateral NIC, the oculomotor nucleus, and the trochlear nucleus. 4. The descending system of NIC projections deployed dense terminal fields in the ipsilateral gigantocellular reticular formation and the paramedian reticular formation of the pons, as well as in the ventromedial and commissural nuclei of the first two spinal cervical segments. It also gave rise to moderate or weak terminal fields in the vestibular complex, the nucleus prepositus hypoglossi, the inferior olive, and the magnocellular reticular formation, as well as cell groups scattered along the paramedian tracts in the pons and the pontine and medullary raphe. 5. The ascending system of NIC projections gave rise to dense terminal fields in the ipsilateral mesencephalic reticular formation and the zona incerta as well as moderate or weak terminal fields in the ipsilateral centromedian and parafascicular thalamic nuclei. It also provided dense bilateral labeling of the rostral interstitial nucleus of the medial longitudinal fasciculus and the fields of Forel, and moderate or weak bilateral labeling of the mediodorsal, central medial, and central lateral nuclei of the thalamus. 6. Models of saccade generation that rely on feedback from the velocity-to-position integrators and include the superior colliculus in their local feedback loop are contradicted because no fibers originating from the NIC traveled to the superior colliculus to deploy terminal fields. 7. Consistent with its morphological and functional diversity, these data indicate that the primate NIC sends signals to a multitude of targets implicated in the control of eye and head movements.

Crescentic and ring-shaped opacities. CT features in two cases of bronchiolitis obliterans organizing pneumonia (BOOP).

Abstract: Two cases of idiopathic bronchiolitis obliterans organizing pneumonia (BOOP) with unusual CT findings are presented. On CT both cases exhibited crescentic and ring-shaped opacities, surrounding areas of groundglass attenuation, and associated with a nodular pattern in one patient and airspace consolidations in the second patient. CT-pathologic correlation disclosed that the central areas of groundglass attenuation corresponded to alveolar septal inflammation, in contrast to the denser periphery where granulomatous tissue in peripheral airspaces predominated. In the broad spectrum of CT findings, BOOP can exhibit specific CT features with regard to the crescentic or ring-shaped opacities with a central groundglass attenuation area. Since these features have not been described in any other disease, they might be characteristic features for the diagnosis of BOOP.

The Complexity of Enzymic Control of Hydrogen Peroxide Concentration May Affect the Regeneration Potential of Plant Protoplasts.

Abstract: Total peroxidase, NADH-peroxidase, ascorbate peroxidase, superoxide dismutase, and catalase activities were measured in tobacco (Nicotiana tabacum) leaves and in regenerating and nonregenerating protoplasts isolated from the same tissue and cultured for 2 weeks. The specific ranges of H2O2 concentration at which the enzymes scavenging the active forms of oxygen may efficiently operate and the activities of those enzymes were determined in an extract from tobacco leaves and in dividing and nondividing tobacco mesophyll protoplasts. The overall H2O2-scavenging enzyme activities were similar in both protoplast populations during the 2 to 3 d of culture. After 3 d, the regenerating protoplasts started to divide and both the antioxidant enzyme activities and the total peroxidase activity increased; in contrast, the viability and the H2O2-scavenging enzyme activities in nonregenerating protoplasts dramatically decreased. Surprisingly, the regenerative potentiality in dividing protoplasts was specifically correlated with a higher NADH-peroxidase activity, which resulted in a net H2O2 accumulation in the cells. Light, which causes the accumulation of active forms of oxygen in photosynthetic organelles, also stimulated catalase and ascorbate peroxidase activities in dividing protoplasts. We suggest that the localization of H2O2 rather than its absolute concentration might be responsible for oxidative stress and that controlled amounts of H2O2 are necessary to allow proper cell-wall reconstitution and the consequent cell division.

Degree of selective constraint as an explanation of the different rates of evolution of gender-specific mitochondrial DNA lineages in the mussel mytilus.

Abstract: Mussels of the genus Mytilus segregate for a maternally transmitted F lineage and a paternally transmitted M lineage of mitochondrial DNA. Previous studies demonstrated that these lineages are older than the species of the M. edulis complex and that the M lineage evolves faster than the F lineage. Here we show that the latter observation also applies to a region of the molecule with no assigned function. Sequence data for the mitochondrial COIII gene and the ``unassigned'' region of the F and M lineages of M. edulis and M. trossulus are used to evaluate various hypotheses that may account for the faster rate of evolution of the M lineage. Tests based on the proportion of synonymous and nonsynonymous substitutions suggest that the M lineage experiences relatively relaxed selection. Further support for this hypothesis comes from an examination of COIII amino acid substitutions at sites defined as either conserved or variable based on the pattern of variation in other mollusks and Drosophila. Most substitutions in the M lineage occur in regions that are also variable among non-Mytilus taxa. We suggest that these differences in selection pressure are a consequence of doubly uniparental mitochondrial DNA transmission in Mytilus.

Parameterless explanation of the non-Arrhenius conductivity in glassy fast ionic conductors.

Abstract: Kincs and Martin [Phys. Rev. Lett. 76, 70 (1996)] have recently discovered a ubiquitous non-Arrhenius temperature dependence of the ionic conductivity in glassy fast ionic conductors when the conductivity exceeds the high level of ${10}^{\ensuremath{-}3}$ to ${10}^{\ensuremath{-}2}{\ensuremath{\Omega}}^{\ensuremath{-}1}{\mathrm{cm}}^{\ensuremath{-}1}$. This important experimental discovery is readily explained and reproduced quantitatively by the coupling scheme without the use of any adjustable parameter.

Extending Promela and Spin for Real Time

Abstract: The efficient representation and manipulation of time information is key to any successful implementation of a verification tool. We extend the syntax and semantics of the higher level specification language Promela to include constructs and statements based on the model of timed Buchi automata [2]. We implement these extensions on top of the verification tool Spin.

Regularity of the free boundary in parabolic phase-transition problems

Abstract: In this paper we start the study of the regularity properties of the free boundary, for parabolic two-phase free boundary problems. May be the best known example of a parabolic two-phase free boundary problem is the Stefan problem, a simplified model describing the melting (or solidification) of a material with a solid-liquid interphase. The concept of solution can be stated in several ways (classical solution, weak so- lution on divergence form, or viscosity solution) and as usual, one would like to prove that the (weak) solutions that may be constructed, are in fact as smooth and classical as possible. Locally, a classical solution of the Stefan problem may be described as following: On the unit cylinder Q1 =B1 “ (-1, 1) we have two complementary domains, ~ and QI\~, separated by a smooth surface S=(OI2)NQ1. In fl and QI\~ we have two smooth solutions, Ul and u2, of the heat equations

Identification and characterization of differentially expressed cDNAs of the vector mosquito, Anopheles gambiae

Abstract: The isolation and study of Anopheles gambiae genes that are differentially expressed in development, notably in tissues associated with the maturation and transmission of the malaria parasite, is important for the elucidation of basic molecular mechanisms underlying vector–parasite interactions. We have used the differential display technique to screen for mRNAs specifically expressed in adult males, females, and midgut tissues of blood-fed and unfed females. We also screened for mRNAs specifically induced upon bacterial infection of larval stage mosquitoes. We have characterized 19 distinct cDNAs, most of which show developmentally regulated expression specificity during the mosquito life cycle. The most interesting are six new sequences that are midgut-specific in the adult, three of which are also modulated by blood-feeding. The gut-specific sequences encode a maltase, a V-ATPase subunit, a GTP binding protein, two different lectins, and a nontrypsin serine protease. The latter sequence is also induced in larvae subjected to bacterial challenge. With the exception of a mitochondrial DNA fragment, the other 18 sequences constitute expressed genomic sequence tags, 4 of which have been mapped cytogenetically.

Environmental innovation, spillovers and optimal policy rules

Abstract: The great majority of the analysis of environmental policy has been undertaken under the assumption that emissions per unit of produced output are constant. Then once environmental policy is introduced, firms either reduce their output or engage in abatement which mainly represents end-of-pipe emissions reduction (e.g., Keeler, Spence and Zeckhauser, 1971; Baumol and Oates, 1988; Barnett, 1980). Recently, however, increasing attention has been directed towards the analysis of environmental policy aimed at reducing unit emissions coefficients through the introduction of environmentally-clean technology. That is, firms are induced by the policy to engage in environmental innovation, or environmental RD Ulph and Ulph, 1994) and by doing so they could reduce emissions without reducing output or undertaking end-of-pipe abatement.

Non-commutative geometry of finite groups

Abstract: A finite set can be supplied with a group structure which can then be used to select (classes of) differential calculi on it via the notions of left-, right- and bicovariance. A corresponding framework has been developed by Woronowicz, more generally for Hopf algebras including quantum groups. A differential calculus is regarded as the most basic structure needed for the introduction of further geometric notions like linear connections and, moreover, for the formulation of field theories and dynamics on finite sets. Associated with each bicovariant first-order differential calculus on a finite group is a braid operator which plays an important role for the construction of distinguished geometric structures. For a covariant calculus, there are notions of invariance for linear connections and tensors. All these concepts are explored for finite groups and illustrated with examples. Some results are formulated more generally for arbitrary associative (Hopf) algebras. In particular, the problem of extension of a connection on a bimodule (over an associative algebra) to tensor products is investigated, leading to the class of `extensible connections'. It is shown that invariance properties of an extensible connection on a bimodule over a Hopf algebra are carried over to the extension. Furthermore, an invariance property of a connection is also shared by a `dual connection' which exists on the dual bimodule (as defined in this work).