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The primary structure of a 2671 bp DNA fragment between the pla gene (encoding plasminogen activator) and the origin of replication of the wild-type Yersinia pestis plasmid pYP358 was determined. Two ORFs of 1074 and 426 bp with opposite transcription polarities were identified on both strands. They encode a 357 aa pesticin activity protein (Pst) and a 141 aa pesticin immunity polypeptide (Pim). A GC-rich palindromic structure located between pst and pim can form a hairpin loop and serve as rho-independent transcription terminator sequences for both genes. The site for the interaction with the LexA repressor of the SOS system was found in another palindromic structure preceding the pst structural gene. A deduced 39.9 kDa Pst polypeptide is devoid of a signal peptide, indicating a Sec-independent mode of export. Pst carries a pentapeptide typical of TonB-dependent colicins (TonB box) that is necessary for the interaction with the yersiniabactin/pesticin receptor and for active energy-dependent transport through the outer membrane. The substitution of the last five C-terminal amino acids did not significantly influence the bactericidal activity of the truncated pesticin. The pesticin lost its ability to kill sensitive bacteria and to bind to a pesticin receptor after deletion of the last 57 C-terminal amino acids. A deduced 16 kDa Pim protein has an N-terminal hydrophobic amino acid stretch with features typical of prokaryotic signal peptides. Pim is a slightly hydrophilic protein with a basic pl. The immunity protein was localized in the periplasmic space and in the outer-membrane fraction after its overexpression under the polymerase T7 promoter. Several other ORFs were identified on the sequenced 2671 bp fragment, but none of them seemed to encode a typical lysis peptide, which is necessary for the release of the pesticin. In the promoter region and in the regions preceding and following the pst operon, the DNA sequence has high (> 70%) identity with other colicin genes. The DNA sequence located 284 bp upstream of the pim gene showed more than 90% similarity to antisense RNA I of the ColE1 replicon. This defined the location of the pYP358 origin of ColE1-type replication.