Divergent evolution can explain how many proteins containing structurally similar domains, which perform a variety of related functions, have evolved from a relatively small number of modules or protein domains. However, it cannot explain how protein domains with similar, but distinguishable, functions and similar, but distinguishable, structures have evolved. Examples of this are the RNA‐binding proteins containing the RNA‐binding domain (RBD), and a newly established protein group, the cold‐shock domain (CSD) protein family. Both protein domains contain conserved RNP motifs on (...) similar single‐stranded nucleic acid‐binding surfaces. Apart from the RNP motifs, which have a similar function, the two families show little similarity in topology or amino acid sequence. This can be considered an interesting example of convergent evolution at the molecular level. Previously, a β‐sheet surface was found to interact with RNA in non‐homologous proteins from yeast, phage and man, revealing that this mode of RNA binding may be a widely recurring theme. (shrink)

One distinguishing feature of vertebrate oocyte meiosis is its discontinuity; oocytes are released from their prophase I arrest, usually by hormonal stimulation, only to again halt at metaphase II, where they await fertilization. The product of the c‐mos proto‐oncogene, Mos, is a key regulator of this maturation process. Mos is a serine‐threonine kinase that activates and/or stabilizes maturation‐promoting factor (MPF), the master cell cycle switch, through a pathway that involves the mitogen‐activated protein kinase (MAPK) cascade. Oocytes arrested at prophase I (...) lack detectable levels of Mos, which must be synthesized from a pool of maternal mRNAs for proper maturation. While Mos is necessary throughout maturation in Xenopus, it seems to be required only for meiosis II in the mouse. The translational activation of c‐mos mRNA at specific times during meiosis requires cytoplasmic polyadenylation. Cis‐ and trans‐acting factors for polyadenylation are, therefore, essential elements of maturation. (shrink)