SEQUENCE-ANALYSIS OF PLASMID PCC5.2 FROM CYANOBACTERIUM SYNECHOCYSTISPCC-6803 THAT REPLICATES BY A ROLLING CIRCLE MECHANISM

The cyanobacterium Synechocystis sp. strain PCC 6803 contains several cryptic plasmids of 2.4, 5.2, and about 50 or 100 kbp. The complete nu cleotide sequence of the 5.2-kbp plasmid, pCC5.2, has been analyzed an d is reported here. This plasmid contains 5214 bp and 53.1% A+T. Six o pen reading frames, ORFs A-F (encoding peptides of larger than 90 amin o acid residues), were located on both strands of pCC5.2. ORF B codes for a potential replication protein containing 971 amino acids, for wh ich there are three homologous proteins encoded by other cyanobacteria l plasmids. ORF C encodes a polypeptide of 93 amino acids which shares homologies with products of two ORFs found in the protein database. C ounterparts of the products of ORF A, D, E, and F could not be found i n the protein data base. Detection of a single-stranded DNA intermedia te during replication of pCC5.2 indicates that this plasmid may also r eplicate by a rolling circle mechanism, as has been reported for pCA2. 4 and pCB2.4 from the same strain of Synechocystis (PCC 6803). (C) 199 7 Academic Press.