FLAGELLAR ASSEMBLY IN CAULOBACTER-CRESCENTUS - A BASAL BODY P-RING NULL MUTATION AFFECTS STABILITY OF THE L-RING PROTEIN

The P- and L-rings are structural components of the flagellar basal bo dy that are positioned in the periplasmic space and outer membrane, re spectively. In order to explore the mechanism of P- and L-ring assembl y, we examined the effect of a null mutation in the gene encoding the P-ring subunit, FlgI, on the expression, stability, and subcellular lo calization of the L-ring subunit, FlgH, in Caulobacter crescentus. Tra nscription of the L-ring gene and synthesis of the L-ring protein were both increased in the P-ring null mutant. However, steady-state L-rin g protein levels were dramatically reduced compared with those of wild type. This reduction, which was not observed in flagellar hook mutant s, was due to a decreased stability of the L-ring protein. The instabi lity of the L-ring protein was apparent throughout the cell cycle of t he P-ring mutant and contrasted with the fairly constant level of L-ri ng protein during the cell cycle of wild-type cells. Low levels of the L-ring protein were detected exclusively in the cell envelope of cell s lacking the P-ring, suggesting that, in the absence of P-ring assemb ly, L-ring monomers are unable to form multimeric rings and are thus s ubject to proteolysis in the periplasm.