Mutagenesis and isolation of Aeromonas hydrophila genes which are required for extracellular secretion

Transposon mutagenesis was used to isolate mutants of Aeromonas hydrophila which were deficient in the production of extracellular proteins. The culture supernatants of two of the mutants were essentially devoid of the proteins normally secreted by the parent strain, despite their continued synthesis. Western immunoblot analysis of one of these proteins indicated that normal signal sequence processing occurred but that normal zymogen activation did not, and cell fractionation experiments indicated that both mutants accumulated the three different extracellular proteins assayed in a position external to the cytoplasmic membrane, presumably in the periplasm. The two mutants differed, however, in that one was lysed during the osmotic shock procedures and also contained severely reduced amounts of two of the major protein components of the outer membrane. The wild-type chromosomal regions into which the transposon had been inserted in the two mutants were cloned. In each case, transconjugants of the mutants containing the corresponding cloned fragment were complemented for the defects in secretion, and one of the mutants was complemented by the heterologous clone as well, suggesting the possibility of an interaction between these two genes or gene products. These results indicate that two separate functions which are required for extracellular secretion were interrupted in the insertion mutants and that one of these is also critically important in the biogenesis of the outer membrane.