F1 histone phosphorylating activity correlates closely in the cell cycle with the behaviour of the mitotic trigger. It is proposed here that the phosphorylation of F1 histone is the initiation step for mitosis.

Control of cell division by very lysine rich histone (F1) phosphorylation.

The extraordinary resistance to disruption shown by the eutherian sperm nucleus is associated with a significant complement of -S-S- crosslinks within the chromatin. The present light and electron microscopic study, in which the spermatozoa of many vertebrates and some invertebrates were exposed to SDS with or without dithiothreitol, demonstrates that the extreme degree of disulfide bonding in the sperm chromatin of eutherian mammals is highly unusual in a comparative sense. In no other species did the sperm nucleus display a resistance to disruption comparable to that in eutherians; on the contrary, in most groups including prototherian and metatherian mammals, such crosslinks are absent or relatively sparse, the greatest degree of crosslinking outside the eutheria being seen in octopus and skate.



The suggestion is advanced that the unusually thiol-rich protamines have evolved as a structural adaptation which, through crosslinking, endows the sperm head with a rigidity of possible importance in the penetration phase of fertilization. This seems likely because (1) any degree of -S-S- determined stability present or inducible in sperm chromatin often appears more intense in or is confined to the anterior region of the nucleus (e.g., newt, snail, turtle and immature eutherian spermatozoa); (2) the zona pellucida, which the fertilizing sperm must penetrate, is an unusually thick, compact and resilient coat in the eutherian ovum in particular; and (3) the occasional observation of a sperm nucleus buckled within the pathway of its penetration into the rabbit zona pellucida suggests that in eutheria this tough vestment presents a relatively rigid barrier to the sperm head.



Its apical location suggests a similar structural role for the -S-S- stabilised perinuclear projection, or “perforatorium” around the leading border of the sperm nucleus in eutherians as well as in some non-eutherian species, such as octopus, snake and turtle.

The occurrence and possible functional significance of -S-S- crosslinks in sperm heads, with particular reference to eutherian mammals.

HISTONE GENE EXPRESSION DURING EMBRyOGENESIS...... . . . . . ...... . . . ...... . . 240 Sea Urchin ......... . . . ... . . . . . . . . . . . . . . . . . 241 Oogenesis.. .. . . .. . . . . .... ......... . .. . . . . . . ........ 242 The u '!fertili zed egg : Maintenan ce o f matern al stores 242 Ma le pronucleus formation: The ro le of CS histones ......... . . . .. . ...... . . 243 Early cleavage : Mobilization o f stored histone mRNAs ....... . . .. . . ... . ... . 243 On set of e mbryoni c histone gene tran scription . . . . ........ . . 244 The transition to late histone gene expression .. . . . . ........ . 245 Sign ificance of the sea urchin histone gene s witch . . . .... ..... . . 245 Drosophila and Xenopus ......... .. . . .. . .. . . . . . ........ . . 247 HISTONE GENE REGULATION DURING THE CELL CYCLE . 249

Expression and Organization of Histone Genes

Limited heterogeneity of the major nonhistone chromosomal proteins.

Biochemical and genetic studies have produced a vast fund of knowledge concerning gene action and regulation in prokaryotes In these organisms the DNA is exposed rather nakedly to the world, protected primarily by the cell membrane In eukaryotes the DNA seems far better shielded, being enmeshed in histone and nonhistone proteins and sequestered behind both the cell and the nuclear membrane These differences have led to a considerable degree of caution in the application of this knowledge of prokaryotes to problems of gene regulation in eukaryotes, and rightly so There are, however, several observations which suggest that higher organisms may have picked up a number of fundamental genetic tricks from their lowly predecessors It has frequently been suggested that eukaryotes must do things differently from prokaryotes, until proven otherwise It may be prudent to reverse this line of thought and suggest that they do things the came until proven different The following similarities suggest this (1) T he basic genetic dogmas concerning DNA replication, transcription, andlation are similar, (2) The genetic code is the same (3) Both systems appear to make use of cyclic AMP as a basic mediator for, humoral or diffusible, signals (4) In both systems DNA synthesis may be controlled at membranes (5) Both make use of different types of RNA polymerase and RNA polymerase cofactors (6) Recent studies of polylysine binding to chromatin suggest the eukaryotic DNA may not be so thoroughly enmeshed in Protein as once thought (7) The visualization of genes in action by electron microscopic techniques intimates that genes are spaced and read in a similar manner And finally, (8) merely because the clustering of related genes is unusual in higher organisms is no reason in itself to totally discard the promoter-operator-repressor concept as a way of regulating single structural genes This system has provided an immense amount of data concerning the manner in which proteins interact with specific DNA sequences to control the attachment and utilization of RNA polymerase It is hard to imagine that eukaryotes, being presented with such a superb mechanism for controlling DNA transcription, would totally discard it and opt for something different It is far more likely that they would build on to this solid foundation

The structure and function of chromatin.

Proteins of interphase and metaphase chromosomes compared.

The relationship between histone and DNA synthesis in HeLa cells.

Chromosomal proteins of interphase HeLa cells.

Synthesis of chromosomal proteins during the maturation of chicken erythrocytes.

Publisher Summary This chapter discusses the various approaches used to decipher the code and evaluates the validity of present codon assignments both in vitro and in vivo. Special attention is devoted to the progress made in elucidating the nature of the signals for initiation and termination of protein synthesis. The coding ambiguity observed in vitro have a significant in vivo role during protein synthesis. Therefore, the effect of the various components involved in protein synthesis on the fidelity of the coding process is examined. The most challenging problem after the deciphering of the genetic code has been presented by the discovery that a transfer RNA species can recognize more than one codon for the same amino acid. Various experiments designed to test the validity and implications of a proposed mechanism for multiple codon recognition are discussed, along with investigations explaining the anticodon and its role in synthetase recognition. The concepts of universality and evolution of the genetic code are re-evaluated in the light of these developments. The need for establishing a direct stereochemical relationship between an amino acid and its coding triplet is obviated by the formulation of the adaptor hypothesis.

Anil Sadgopal

Papers

Proteins of interphase and metaphase chromosomes compared.

The relationship between histone and DNA synthesis in HeLa cells.

Chromosomal proteins of interphase HeLa cells.

Synthesis of chromosomal proteins during the maturation of chicken erythrocytes.

The genetic code after the excitement.