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. Ho mologues of Outs have been described in two other bacteria. Klebsiella oxyt oca and Aeromonas hydrophila, but their requirement in the secretion proces s seems to be different. Study of Outs topology with the BlaM topology prob e suggests that it is an inner-membrane protein with a large periplasmic do main. However, fractionation experiments indicate that it could be associat ed with the outer membrane through its C-terminal part. The secretion defic iency of an Erw. chrysanthemi outs mutant can be reversed by the addition o f an inducer of the kdgR regulon. it was shown that this effect results fro m the increased expression of the secretin OutD and that secretion can be r estored in an outB mutant by introducing the outD gene on a plasmid. Severa l experiments suggest an interaction between Outs and OutD. In Erw. chrysan themi, the presence of OutD stabilizes Outs. OutD expressed in Escherichia coli can be protected from proteolytic degradation by the coexpression of O uts. This effect does not require the N-terminal. transmembrane segment of outB. Outs can be cross-linked with Outs by formaldehyde. These results ind icate that Outs could act with OutD in the functioning of the Out secretion machinery.