Disruption ofcagA, the apoprotein gene of chromoprotein antibiotic C-1027, eliminates holo-antibiotic production, but not the cytotoxic chromophore

Abstract

C-1027 is a chromoprotein of the nine-membered enediyne antitumour antibiotic family, comprising apoprotein to stabilize and transport the enediyne chromophore. The disruption of apoprotein gene cagA within the C-1027 biosynthetic gene cluster abolished C-1027 holo-antibiotic production detected by an antibacterial assay, as well as the expression of the apoprotein and C-1027 chromophore extracted following protein precipitation of the culture supernatant. Complementation of the cagA-disrupted mutant AKO with the intact cagA gene restored C-1027 production, suggesting that cagA is indispensable for holo-antibiotic production. Overexpression of cagA in the wild-type strain resulted in a significant increase in C-1027 production as expected. Surprisingly, electrospray ionization (ESI)-MS and ESI-MS/MS analyses suggested that the AKO mutant still produced the C-1027 enediyne chromophore [m/z=844 (M+H)(+)] and its aromatized product [m/z=846 (M+H)(+)]. Consistent with this, the results from gene expression analysis using real-time reverse transcriptase-PCR showed that transcripts of the positive regulator sgcR3 and the structural genes sgcA1, sgcC4, sgcD6 and sgcE were readily detected in the AKO mutant as well as in the wild-type and the complementation strain. These results provided, for the first time, evidence suggesting that the apoprotein of C-1027 is not essential in the self-resistance mechanism for the enediyne chromophore.