The 5.2-kb ColJs plasmid of a colicinogenic strain of Shigella sonnei (colicin type 7) was isolated and sequenced. pColJs was partly homologous to pColE1 and to pesticin-encoding plasmid pPCP1, mainly in the rep, mob, and cer regions. A 1.2-kb unique region of pColJs showed significantly different G+C content (34%) compared to the rest of pColJs (53%). Within the unique region, seven open reading frames (ORFs) were identified. ORF94 was shown to code for colicin Js activity (cja), a 94-amino-acid polypeptide (molecular mass, 10.4 kDa); ORF129 (cji) was shown to code for the 129-amino-acid colicin Js immunity protein (molecular mass, 14.3 kDa); and ORF65 was shown to be involved in colicin Js release by producer bacteria (cjl) coding for a 65-amino-acid polypeptide (molecular mass, 7.5 kDa). In contrast to the gene order in other colicin operons, the cjl gene was found upstream from cja. Moreover, the promoter upstream from cjl was similar to promoters described upstream from several colicin activity genes. The cji gene was found to be located downstream from cja with a transcription polarity opposite to that of the cjl and cja genes. The cja, cji, and cjl genes were not similar to other known colicin genes. Colicin Js was purified as an inactive fusion protein with an N-terminal histidine tag. Activity of the purified fusion form of colicin Js was restored after cleavage of the amino acids fused to the colicin Js N terminus.