STRUCTURE AND EXPRESSION OF THE GENE ENCODING RIBOSOMAL-PROTEIN-S1 FROM THE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC-6301 - STRIKING SEQUENCE SIMILARITY TO THE CHLOROPLAST RIBOSOMAL PROTEIN-CS1

We isolated a 38 kDa ssDNA-binding protein from the unicellular cyanob acterium Synechococcus sp. strain PCC 6301 and determined its N-termin al amino acid sequence. A genomic clone encoding the 38 kDa protein wa s isolated by using a degenerate oligonucleotide probe based on the am ino acid sequence. The nucleotide sequence and predicted amino acid se quence revealed that the 38 kDa protein is 306 amino acids long and ho mologous to the nuclear-encoded 370 amino acid chloroplast ribosomal p rotein CS1 of spinach (48% identity), therefore identifying it as ribo somal protein (r-protein) S1. Cyanobacterial and chloroplast S1 protei ns differ in size from Escherichia coli r-protein S1 (557 amino acids) . This provides an additional evidence that cyanobacteria are closely related to chloroplasts. The Synechococcus gene rps1 encoding S1 is lo cated 1.1 kb downstream from psbB, which encodes the photosystem II P6 80 chlorophyll a apoprotein. An open reading frame encoding a potentia l protein of 168 amino acids is present between psbB and rps1 and its deduced amino acid sequence is similar to that of E. coli hypothetical 17.2 kDa protein. Northern blot analysis showed that rps1 is transcri bed as a monocistronic mRNA.