CHARACTERIZATION OF A GENE LOCUS FROM ERWINIA-AMYLOVORA WITH REGULATORY FUNCTIONS IN EXOPOLYSACCHARIDE SYNTHESIS OF ERWINIA SPP

In a genomic library of Erwinia amylovora, a locus has been identified that can suppress an Erwinia stewartii rcsA mutant. In addition, the locus induced a mucoid sticky phenotype of colonies in a wild-type str ain of Erwinia stewartii and increased exopolysaccharide synthesis in several species of bacteria belonging to the genus Erwinia. An open re ading frame was identified at this locus encoding a 225 amino acid pro tein that contained a helix-turn-helix motif typical of transcriptiona l regulators. The corresponding gene was subsequently named rcsV (regu lator of capsular synthesis affecting viscosity). A mutant of rcsV in wild-type Erwinia amylovora had no detectable phenotype and produced t ypical levels of amylovoran under laboratory conditions. The rcsV gene on a high copy number plasmid under the control of its own promoter d id not alter amylovoran production, in contrast to in-frame fusions of the structural gene in expression vectors. Since even the lac promote r was inert in the expression of rcsV a DNA-binding protein could inhi bit transcription of the gene in Erwinia amylovora. On the other hand, an Erwinia amylovora rcsA mutant was suppressed by rcsV when its prom oter was replaced and the structural gene fused in-frame with lacZ' or malE. Northern blots, with total RNA from Erwinia amylovora, or promo ter analysis using the GUS reporter gene did not show expression of rc sV in Erwinia amylovora, although primer extension analysis did. RcsV could be a component involved in the regulation of amylovoran synthesi s, and gene expression may require an unknown external signal during t he life cycle or pathogenesis of Erwinia amylovora.