Lysis protein encoded by plasmid ColA-CA31. Gene sequence and export

A gene, cal, coding for a polypeptide needed for the release of colicin A from Escherichia coli cells has been identified by transposon insertion. The cal gene was located on the ColA plasmid map adjacent to cai, the gene coding for colicin A immunity protein, and therefore 592 bases downstream from caa, the structural gene for colicin A. Transcription of cal is in the same direction as caa, that is in the opposite direction to cai. Its sequence has been determined and the predicted amino acid composition features a basic N-terminal end followed by a serie of hydrophobic residues similar to the signal sequence in precursors of exported proteins. The C-terminal part also contains a core of hydrophobic residues. The overall amino acid sequence of the cal protein is homologous to that of lytic proteins encoded by the related plasmids pColE1, and pCloDF13. The cal protein has been identified on urea-SDS-polyacrylamide gels by selective labelling with various radioactive amino acids and its synthesis is co-induced with that of colicin A. The cal protein undergoes slow processing with loss of the N-terminal "signal" region and the mature form is released into the medium together with colicin A.