A group of proteins containing a conserved ribonucleoprotein consensus sequence (RNP-CS)-type RNA-binding domain (CS-RBD) of approximately 80 amino acids is present in eukaryotic cells and binds specifically to a wide variety of RNA molecules. We have isolated 12 kDa single-stranded DNA binding proteins from the unicellular cyanobacterium Synechococcus 6301. The amino-terminal sequence was determined and two distinct genomic clones were isolated from a Synechococcus 6301 genomic library. Sequence analysis revealed that two closely related proteins contain a single CS-RBD of 82 amino acids and are named as 12RNP1 and 12RNP2. Both of the CS-RBDs share the highest amino acid identity with those of chloroplast ribonucleoproteins (40-51%). The 12RNP proteins were expressed in Escherichia coli bearing plasmids encoding glutathione S-transferase/12RNP fusion proteins and subjected to in vitro nucleic acid-binding assay. Both 12RNP1 and 12RNP2 bind to RNA homopolymers poly(U) and poly(G), indicating that they might be RNA-binding proteins. This is the first example of such proteins in prokaryotes. The 12RNP1 and 12RNP2 genes are transcribed as monocistronic mRNAs and the steady-state mRNA level of 12RNP1 is over 20-fold than that of 12RNP2. Due to the easiness of genetic manipulations the cyanobacterium will provide an excellent system to analyze the function of not only cyanobacterial but also plant RNA-binding proteins.