- The minimal gene set member msrA, encoding peptide methionine sulfoxide reductase, is a virulence determinant of the plant pathogen Erwinia chrysanthemi. Hassouni, M.E., Chambost, J.P., Expert, D., Van Gijsegem, F., Barras, F. Proc. Natl. Acad. Sci. U.S.A. (1999)