MOLECULAR-CLONING AND CHARACTERIZATION OF THE CTPA GENE ENCODING A CARBOXYL-TERMINAL PROCESSING PROTEASE - ANALYSIS OF A SPONTANEOUS PHOTOSYSTEM-II-DEFICIENT MUTANT STRAIN OF THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6803

A nitrofurantoin enrichment technique was used to isolate a spontaneou s photosynthesis-deficient mutant strain of the unicellular cyanobacte rium Synechocystis sp. PCC 6803. This mutant, SK18, lacked any photosy stem II (PSII) activity, but had normal photosystem I. The SK18 mutant strain could not be complemented with known genes encoding various st ructural proteins of PSII, but could be complemented with a recombinan t plasmid pSL523 containing a 1.4-kilobase pair EcoRI fragment of the chromosomal DNA from wild-type Synechocystis 6803 cells. Determination of the nucleotide sequence of this DNA fragment revealed a previously unidentified open reading frame (ORF) encoding a 427-residue-long pol ypeptide. Hydrophobicity analysis of the amino acid sequence suggested that this protein is largely hydrophilic. A stretch of the first 31 a mino-terminal residues of the polypeptide resembled a bacterial signal peptide and may be responsible for the translocation of this protein to the lumen space of the thylakoid membranes. The spontaneous mutatio n in the SK18 strain was identified to be a single nucleotide change i ntroducing a premature termination codon in this ORF. The predicted se quence of the encoded protein showed significant similarity to that of the Pre protein, a carboxyl-terminal processing protease in Escherich ia coli. We suggest that the cyanobacterial protein encoded by ORF427 is a similar processing protease and name the gene ctpA (carboxyl-term inal processing protease).