Rem, a new transcriptional activator of motility and chemotaxis in Sinorhizobium meliloti.

The expression of 51 known genes clustered in the flagellar regulon of Sinorhizobium meliloti is organized as a three-class hierarchy: class IA comprises the master regulatory genes, visN and visR; class II, controlled by VisNR, comprises flagellar assembly and motility genes; and class III comprises flagellin and chemotaxis genes requiring class II for expression. The expression of visN-visR is constitutive throughout growth, whereas that of class II and class III genes is limited to exponential growth. A new OmpR-like, 25-kDa transcription factor, Rem, whose synthesis is confined to exponential growth, was shown to positively control swimming motility.