SoxR-dependent response to oxidative stress and virulence of Erwinia chrysanthemi: the key role of SufC, an orphan ABC ATPase

Erwinia chrysanthemi causes soft-rot disease in a great variety of plants. In addition to the depolymerizing activity of plant cell wall-degrading enz ymes, iron acquisition and resistance to oxidative stress contribute greatl y to the virulence of this pathogen. Here, we studied the pin10 locus origi nally thought to encode new virulence factors. The sequence analysis reveal ed six open reading frames that were homologous to the Escherichia coil suf A, sufB, sufC, sufD, sufS and sufE genes. Sequence similarity searching pre dicted that (i) SufA, SufB, SufD, SufS and SufE proteins are involved in ir on metabolism and possibly in Fe-S cluster assembly; and (ii) SufC is an AT Pase of an ABC transporter. The reverse transcription-polymerase chain reac tion procedure showed that the sufABCDSE genes constitute an operon, Expres sion of a sufB::uidA fusion was found to be induced in iron-deficient growt h conditions and to be repressed by the iron-sensing Fur repressor. Each of the six suf genes was inactivated by the insertion of a cassette generatin g a non-polar mutation. The intracellular iron level in the sufA, sufS, suf C, sufS and sufE mutants was higher than in the wild type, as assessed by i ncreased sensitivity to the iron-activated antibiotic streptonigrin. In add ition, inactivation of sufC and sufD led to increased sensitivity to paraqu at. Virulence tests showed that sufA and sufC mutants exhibited reduced abi lity to cause maceration of chicory leaves, whereas a functional sufC gene was necessary for the bacteria to cause systemic invasion of Saintpaulia io nantha. The E. coli sufC homologue was inactivated by reverse genetic. This mutation was found to modify the soxR-dependent Induction of soxS gene exp ression. We discuss the possibility that SufC is a versatile ATPase that ca n associate either with the other Suf proteins to form a Fe-S cluster-assem bling machinery or with membrane proteins encoded elsewhere in the chromoso me to form an Fe-S ABC exporter. Overall, these results stress the importan ce of the connection between iron metabolism and oxidative stress during th e early steps of infection by E. chrysanthemi.