Genetic and biochemical analysis of protein export from Xanthomonas campestris

Xanthomonas campestris pv. campestris, a Gram-negative phytopathogen, produces a number of extracellular enzymes which can degrade components of the host plant cell. Some non-pathogenic mutants, derived by chemical mutagenesis, were found to be defective in the export but not the synthesis of a number of these enzymes. The pathogenicity and export lesions in one such mutant, strain 8288, could be complemented by a cosmid clone pIJ3000 from the Xanthomonas library. Mutagenesis of pIJ3000 with the transposon Tn5 followed by recombination into the corresponding region of the chromosome has revealed a cluster of 6 to 8 genes whose function is required for enzyme export. Sequence analysis of part of the cluster has revealed two open reading frames that would encode proteins with extensive hydrophobic domains. Export-defective mutants retain the normally exported enzymes in the periplasmic space. These forms have the same molecular weight as the extracellular forms, suggesting that the signal sequence has been properly processed. The results are consistent with a mechanism of sequential translocation across cytoplasmic and outer membrane via the periplasm. The second translocation step may be mediated by the products of export genes. This may be a common export mechanism amongst Gram-negative bacteria but other mechanisms do exist, sometimes in parallel in the same cell, and these are briefly reviewed.