Inhibition of colicin synthesis by the antibiotic globomycin

The antibiotic globomycin, an inhibitor of LspA (the lipoprotein signal peptidase), inhibited synthesis of colicin by Escherichia coli cells grown in rich medium. This inhibition was stronger in cells with mutation(s) within either the colicin operon, which is located on a plasmid, or the host chromosome. This phenotype was called Gbc (globomycin blocks colicin synthesis). The Gbc phenotype was affected by growth conditions since it was partially or totally suppressed in cells subjected to high temperatures, treated with sodium azide, or grown in minimal medium. The Gbc phenotype observed with colicin-A-producing cells was more severe in strains carrying plasmids with a deletion within caa (the first gene of the colicin A operon), which encodes colicin A, than in cells with wild-type caa gene. The Gbc phenotype was alleviated by a null mutation in the degP gene encoding in the DegP/HtrA protease, abolished by a null mutation in the lpp gene encoding the murein-lipoprotein, and enhanced by a mutation in the pldA gene encoding the outer membrane phospholipase A. Transcription of the colicin A operon was blocked in cells exhibiting the Gbc phenotype as evidenced by rifampicin treatment of induced cells. This phenotype suggests that either a lipoprotein or a protein involved in lipoprotein metabolism might be involved in the regulation of the expression of the colicin operons and that the colicin A structural gene might play a role in the regulation of transcription of the colicin A operon.