G. Ramakrishnan et al., MULTIPLE STRUCTURAL PROTEINS ARE REQUIRED FOR BOTH TRANSCRIPTIONAL ACTIVATION AND NEGATIVE AUTOREGULATION OF CAULOBACTER-CRESCENTUS FLAGELLAR GENES, Journal of bacteriology, 176(24), 1994, pp. 7587-7600
The periodic and sequential expression of flagellar (fla) genes in the
Caulobacter crescentus cell cycle depends on their organization into
levels I to IV of a regulatory hierarchy in which genes at the top of
the hierarchy are expressed early in the cell cycle and are required f
or the later expression of genes below them. In these studies, we have
examined the regulatory role of level II fliF operon, which is locate
d near the top of the hierarchy. The last gene in the fliF operon, flb
D, encodes a transcriptional factor required for activation of sigma(5
4)-dependent promoters at levels III and IV and negative autoregulatio
n of the level II fliF promoter. We have physically mapped the fliF op
eron, identified four new genes in the transcription unit, and determi
ned that the organization of these genes is 5'-fliF-fliG-flbE-fliN-flb
D-3'. Three of the genes encode homologs of the MS ring protein (FliF)
and two switch proteins (FliG and FliN) of enteric bacteria, and the
fourth encodes a predicted protein (FlbE) without obvious similarities
to known bacterial proteins. We have introduced nonpolar mutations in
each of the open reading frames and shown that all of the newly ident
ified genes (fliF, fliG, flbE, and fliN) are required in addition flbD
for activation of the sigma(54)-dependent flgK and flbG promoters at
level III. In contrast, fliF, fliG, and flbE, but not fliN, are requir
ed in addition flbD for negative autoregulation of the level II fliF p
romoter. The simplest interpretation of these results is that the requ
irements of FlbD in transcriptional activation and repression are not
identical, and we speculate that FlbD function is subject to dual or o
verlapping controls. We also discuss the requirement of multiple struc
tural genes for regulation of levels II and III genes and suggest that
fla gene expression in C. crescentus mag be coupled to two checkpoint
s in flagellum assembly.