DIFFERENTIAL EFFECT OF DSBA AND DSBC MUTATIONS ON EXTRACELLULAR ENZYME-SECRETION IN ERWINIA-CHRYSANTHEMI

An Erwinia chrysanthemi gene able to complement an Escherichia coli ds bA mutation has been cloned and sequenced. This gene codes for a perip lasmic protein with disulphide isomerase activity that has 69% identit y and 94% similarity with the E. coli DsbA protein. An E. chrysanthemi dsbA-uidA fusion mutant has been constructed. dsbA expression seems t o be constitutive. This mutant has multiple phenotypes resulting from the absence of disulphide bond formation in periplasmic and secreted p roteins. Pectate lyases and the cellulase EGZ are rapidly degraded in the periplasm of the dsbA mutant. E. chrysanthemi synthesizes another periplasmic protein with disulphide isomerase activity, namely DsbC. T he dsbC gene introduced on a multicopy plasmid in a dsbA mutant was on ly partially able to restore EGZ secretion, indicating that even if Ds bA and DsbC possess disulphide oxydoreductase activity, they are not c ompletely interchangeable. Moreover, pectate lyases expressed in an E. coli dsbA mutant were very instable but their stability was unaffecte d in a dsbC mutant. These results indicate that DsbA and DsbC could ha ve different substrate specificities.