Many microorganisms prefer easily metabolizable carbon sources over alternative, less readily metabolized carbon sources. One of the mechanisms to achieve this is repression of the synthesis of enzymes related to catabolism of the alternative carbon sources, i.e. carbon repression. It is now clear that in Aspergillus nidulans and Aspergillus niger the repressor protein CREA plays a major role in carbon repression. CREA inhibits transcription of many target genes by binding to specific sequences in the promoter of these genes. Unfortunately there is little information on other components of the signalling pathway that triggers repression by CREA. In this review we summarize the current understanding of carbon repression in aspergilli.

Carbon repression in aspergilli

Major insights into sexual development and cryptic sexuality within filamentous fungi have been gained from investigations using Aspergillus species Here, an overview is first given into sexual morphogenesis in the aspergilli, describing the different types of sexual structures formed and how their production is influenced by a variety of environmental and nutritional factors It is argued that the formation of cleistothecia and accessory tissues, such as Hulle cells and sclerotia, should be viewed as two independent but co-ordinated developmental pathways Next, a comprehensive survey of over 75 genes associated with sexual reproduction in the aspergilli is presented, including genes relating to mating and the development of cleistothecia, sclerotia and ascospores Most of these genes have been identified from studies involving the homothallic Aspergillus nidulans, but an increasing number of studies have now in addition characterized 'sex-related' genes from the heterothallic species Aspergillus fumigatus and Aspergillus flavus A schematic developmental genetic network is proposed showing the inter-relatedness between these genes Finally, the discovery of sexual reproduction in certain Aspergillus species that were formerly considered to be strictly asexual is reviewed, and the importance of these findings for cryptic sexuality in the aspergilli as a whole is discussed

Sexual development and cryptic sexuality in fungi: insights from Aspergillus species

https://mmbr.asm.org/content/mmbr/45/3/462.full.pdf

Adenosine 3',5'-phosphate in fungi.

The role of zinc on the primary metabolism of Aspergillus parasiticus in relation to aflatoxin biosynthesis was studied. Zinc deficiency impaired growth and metabolism of nucleic acid and protein while stimulating that of lipid. The activities of enzymes of glycolytic cycle were affected indicating that these are zinc dependent enzymes. Impaired glycolytic cycle tends to reduce the level of primary metabolites like pyruvate, citrate and oxaloacetate which trigger aflatoxin formation. During stationary phase these precursors accumulate as these are not used up for the synthesis of secondary metabolites (aflatoxins) due to zinc deficiency. ATP and energy-charge (E.C.) which are important for various physiological processes like growth are reduced in zinc deficiency. Also zinc deficiency results in low levels of AMP which favour synthesis of lipid. Inorganic phosphate accumulated due to zinc deficiency and this may be unfavourable for aflatoxin synthesis.

https://mmbr.asm.org/content/mmbr/41/4/822.full.pdf

Biosynthesis of aflatoxins.

A comparison of citric acid fermentations in manganese-deficient and manganese-containing media showed that manganese strongly influences idiophase metabolism In the presence of manganese, cell growth increases, sugar consumption is diminished and acidogenesis decreases drastically An investigation of the key enzymes of glycolysis, the pentosephosphate pathway, TCA-cycle, nitrogen metabolism, and gluconeogenesis indicated that manganese deficiency was accompanied by a repression of anabolic and TCA-cycle-enzymes with the exception of citrate synthase The activity of this enzyme and the enzymes of glycolysis paralleled the sugar consumption rate In the presence of manganese, no repression of enzyme synthesis was observed Activities of 2-oxoglutarate dehydrogenase and isocitrate lyase could not be detected in either case The results support the hypothesis that manganese deficiency mainly affects the operation of biosynthetic reactions inAspergillus niger, thus leading to an overflow of citric acid as an end product of glycolysis

Influence of manganese on enzyme synthesis and citric acid accumulation inAspergillus niger

Aspergillus nidulans was grown on media with added amounts of manganese ranging from 0–2.5 μM. Manganese deficiency prevented cleistothecium development, although good vegetative growth was retained. Subsequent analysis of the mycelium produced under Mn2+ deficient growth revealed that α-1,3 glucan, the main carbon and energy source for fructification, was virtually absent from the cell wall. Several enzymes related to cell wall composition were investigated. β-1,3 glucanase, and very remarkably, α-1,3 glucanase reached about the same activity on the Mn2+ deficient and sufficient media, but amylase and protease were about 60 and 75% lower respectively on the Mn2+ deficient media and the correlation of these findings is discussed.

Sexual differentiation in Aspergillus nidulans

The role of cyclic adenosine monophosphate (cAMP) during growth and development of Aspergillus nidulans was investigated. In normal cultures the highest amount of cAMP, expressed on a dry weight basis, was found after 24 h of growth when still more than 5% glucose was present in the medium. After depletion of the medium even a slight fall in cAMP was noted. Glucose concentrations ranging from 0.5–12% resulted in a slight decrease in the amount of cAMP as measured after 24 h of growth.

The effect of glucose and manganese on adenosine-3′, 5′-monophosphate levels during growth and differentiation of Aspergillus nidulans

Aspergillus nidulans was completely devoid of fruit bodies when grown on manganese deficient cultures. This result was shown earlier to be due to a lack of alpha-1,3 glucan in the cell wall. Several enzymes of carbon and nitrogen metabolism were investigated in an attempt to explain the absence of this reserve material. Synthesis of glucose-6-phosphate dehydrogenase, phosphoglucoisomerase and aldolase, were not strongly affected by manganese deficiency. However, phosphoglucomutase showed only 60% of the activity of the control cultures and it was argued that this was connected with the low amounts of alpha-1,3 glucan synthesized. Malate dehydrogenase was the enzyme the least affected by manganese deficiency and the two to threefold higher activity measured after glucose depletion might indicate the induction of the glyoxylate cycle. An impaired glutamine synthetase could explain the increase in activity observed for NAD-glutamine dehydrogenase.

Sexual differentiation in Aspergillus nidulans: the requirement for manganese and the correlation between phosphoglucomutase and the synthesis of reserve material.

Ben J. M. Zonneveld

Papers

Sexual differentiation in Aspergillus nidulans

The effect of glucose and manganese on adenosine-3′, 5′-monophosphate levels during growth and differentiation of Aspergillus nidulans

Sexual differentiation in Aspergillus nidulans: the requirement for manganese and the correlation between phosphoglucomutase and the synthesis of reserve material.