Conservation of structure and cold-regulation of RNA-binding proteins in cyanobacteria: probable convergent evolution with eukaryotic glycine-rich RNA-binding proteins

The rbp gene family of the cyanobacterium Anabaena variabilis strain M3 con sists of eight members that encode small RNA-binding proteins containing a single RNA recognition motif (RRM), Similar genes are found in the genomes of Synechocystis sp, PCC6803, Helicobacter pylori and Treponema pallidum, b ut are absent from the other completely sequenced prokaryotic genomes, The expression of the rbp genes of Anabaena is induced by low temperature, with the exception of the rbpD gene, We found four stretches of conserved seque nces in the 5'-untranslated region of the cyanobacterial rbp genes that are known to be induced by low temperature. The cold-regulated Rbp proteins co ntain a short C-terminal glycine-rich domain, In this respect, these protei ns are similar to plant and mammalian glycine-rich RNA-binding proteins (GR Ps), which also contain a single RRM domain with a C-terminal glycine-rich domain and are highly expressed at low temperature. Detailed phylogenetic a nalysis showed, however, that the cyanobacterial Rbp proteins and the eukar yotic GRPs do not belong to a single lineage, but that the glycine-rich dom ains are likely to have been added independently. The cold-regulation of bo th types of proteins is also likely to have evolved independently. Furtherm ore, the chloroplast RNA-binding proteins are not likely to have originated from the Rbp proteins of endosymbiont cyanobacterium, but are supposed to have diverged from the GRPs, These results suggest that the cyanobacterial Rbp proteins and the eukaryotic GRPs are similar in both structure and regu lation, but that this apparent similarity has resulted from convergent evol ution.