A CASE OF CONVERGENT EVOLUTION OF NUCLEIC-ACID BINDING MODULES

Divergent evolution can explain how many proteins containing structura lly similar domains, which perform a variety of related functions, hav e evolved from a relatively small number of modules or protein domains . However, it cannot explain how protein domains with similar, but dis tinguishable, functions and similar, but distinguishable, structures h ave evolved. Examples of this are the RNA-binding proteins containing the RNA-binding domain (RED), and a newly established protein group, t he cold-shock domain (CSD) protein family. Both protein domains contai n conserved RNP motifs on similar single-stranded nucleic acid-binding surfaces. Apart from the RNP motifs, which have a similar function, t he two families show little similarity in topology or amino acid seque nce. This can be considered an interesting example of convergent evolu tion at the molecular level. Previously, a P-sheet surface was found t o interact with RNA in non-homologous proteins from yeast, phage and m an, revealing that this mode of RNA binding may be a widely recurring theme.