SPINACH CSP41, AN MESSENGER-RNA-BINDING PROTEIN AND RIBONUCLEASE, IS HOMOLOGOUS TO NUCLEOTIDE-SUGAR EPIMERASES AND HYDROXYSTEROID DEHYDROGENASES

Spinach CSP41 is part of a protein complex that hinds to the 3' untran slated region (UTR) of petD precursor-mRNA, a chloroplast gene encodin g subunit IV of the cytochrome b6/f complex, CSP41 cleaves the 3'-UTR of petD mRNA within the stem-loop structure, suggesting a key role in the control of chloroplast mRNA stability. We discovered that CSP41 is homologous to nucleotide-sugar epimerases and hydroxysteroid dehydrog enases while seeking distant homologs of these enzymes with a hidden M arkov model-based search of Genpept. This analysis identified Synechoc ystis ORF, Accession 1652543 as a homolog, Subsequent analyses show th at spinach CSP41 and Arabidopsis thaliana 2765081 are homologous to th e Synechocystis ORF. Information from the solved 3D structures of epim erases and dehydrogenases and our motif analysis of these enzymes is u sed to predict domains on CSP41 that are important in binding and meta bolism of mRNA. Cyanobacteria are among the earliest life forms, indic ating that the divergence from a common ancestor of nucleotide-sugar e pimerases and an mRNA binding protein with ribonuclease activity was a ncient. (C) 1998 Academic Press.