High-level expression of the colicin A lysis protein

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Abstract
Two plasmids that overproduce the colicin A lysis protein, Cal, are described. Plasmid AT1 was constructed by a deletion in the colicin A operon, which placed thecal gene near a truncatedcaa gene in such a way that both gene products were synthesized at high levels following induction. Plasmid Ck4 was constructed by insertion of thecal gene downstream from thetac promoter of an expression vector. Overproduction of Cal was obtained after mitomycin C induction of pAT1 cells and after IPTG induction of pCK4 cells. The kinetics of Cal synthesis were examined with [35S] methionine and [2-3H] glycerol inlpp orlpp + host strains. Each of the steps of the lipid modification and maturation pathway of Cal was demonstrated. The modified precursor form of overproduced Cal was not chased as efficiently as when it is produced in pColA cells. After treatment with globomycin, a significant amount of this modified precursor form accumulated and was degraded with time into smaller acylated proteins, but without release of the signal peptide. Release of cellular proteins and quasi-lysis were observed after about 1 hour of induction for cells containing either plasmid. In addition, in Cal-overproducing cells, the rate of quasi-lysis was increased but not its extent. InpldA cells, quasi-lysis was reduced but not abolished. Lethality of the Cal induction in the overproducing cells was in the same range as that in wild-type cells.