Overproduction of the secretin OutD suppresses the secretion defect of an Erwinia chrysanthemi outB mutant

OutB is a component of the Erwinia chrysanthemi Out secretion machinery. Homologues of Outs have been described in two other bacteria. Klebsiella oxytoca and Aeromonas hydrophila, but their requirement in the secretion process seems to be different. Study of Outs topology with the BlaM topology probe suggests that it is an inner-membrane protein with a large periplasmic domain. However, fractionation experiments indicate that it could be associated with the outer membrane through its C-terminal part. The secretion deficiency of an Erw. chrysanthemi outs mutant can be reversed by the addition of an inducer of the kdgR regulon. it was shown that this effect results from the increased expression of the secretin OutD and that secretion can be restored in an outB mutant by introducing the outD gene on a plasmid. Several experiments suggest an interaction between Outs and OutD. In Erw. chrysanthemi, the presence of OutD stabilizes Outs. OutD expressed in Escherichia coli can be protected from proteolytic degradation by the coexpression of Outs. This effect does not require the N-terminal. transmembrane segment of outB. Outs can be cross-linked with Outs by formaldehyde. These results indicate that Outs could act with OutD in the functioning of the Out secretion machinery.