Mohr C D, Jenal U, Shapiro L
Department of Developmental Biology, Beckman Center, Stanford University School of Medicine, California 94305-5427, USA.
J Bacteriol. 1996 Feb;178(3):675-82. doi: 10.1128/jb.178.3.675-682.1996.
The P- and L-rings are structural components of the flagellar basal body that are positioned in the periplasmic space and outer membrane, respectively. In order to explore the mechanism of P- and L-ring assembly, we examined the effect of a null mutation in the gene encoding the P-ring subunit, FlgI, on the expression, stability, and subcellular localization of the L-ring subunit, FlgH, in Caulobacter crescentus. Transcription 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-ring protein levels were dramatically reduced compared with those of wild type. This reduction, which was not observed in flagellar hook mutants, was due to a decreased stability of the L-ring protein. The instability of the L-ring protein was apparent throughout the cell cycle of the P-ring mutant and contrasted with the fairly constant level of L-ring protein during the cell cycle of wild-type cells. Low levels of the L-ring protein were detected exclusively in the cell envelope of cells lacking the P-ring, suggesting that, in the absence of P-ring assembly, L-ring monomers are unable to form multimeric rings and are thus subject to proteolysis in the periplasm.
P环和L环是鞭毛基体的结构成分，分别位于周质空间和外膜中。为了探究P环和L环的组装机制，我们研究了编码P环亚基FlgI的基因发生无效突变对新月柄杆菌中L环亚基FlgH的表达、稳定性和亚细胞定位的影响。在P环缺失突变体中，L环基因的转录和L环蛋白的合成均增加。然而，与野生型相比，稳态L环蛋白水平显著降低。这种降低在鞭毛钩突变体中未观察到，是由于L环蛋白稳定性降低所致。L环蛋白的不稳定性在P环突变体的整个细胞周期中都很明显，这与野生型细胞在细胞周期中L环蛋白水平相当恒定形成对比。仅在缺乏P环细胞的细胞膜中检测到低水平的L环蛋白，这表明在没有P环组装的情况下，L环单体无法形成多聚环，因此在周质中易被蛋白酶水解。
