POSTTRANSCRIPTIONAL REGULATION OF CAULOBACTER FLAGELLIN GENES BY A LATE FLAGELLUM ASSEMBLY CHECKPOINT

Flagellum formation in Caulobacter crescentus requires ca. 50 flagella r genes, most of which belong to one of three classes (II, III, or IV) . Epistasis experiments suggest that flagellar gene expression is coor dinated with flagellum biosynthesis by two assembly checkpoints. Compl etion of the M/S ring-switch complex is required for the transition fr om class a to class III gene expression, and completion of the basal b ody-hook structure is required for the transition from class III to cl ass IV gene expression. In studies focused on regulation of the class IV flagellin genes, we have examined fljK and fljL expression in a lar ge number of flagellar mutants by using transcription and translation fusions to lacZ, nuclease S1 assays, and measurements of protein stabi lity. The fljK-lacZ and fljL-lacZ transcription fusions were expressed in all class III flagellar mutants, although these strains do not mak e detectable 25- or 27-kDa flagellins. The finding that the fljK-lacZ translation fusion was not expressed in the Same collection of class I II mutants confirmed that fljK is regulated posttranscriptionally. The requirement of multiple class III genes for expression of the fljK-la cZ fusion suggests that completion of the basal body-hook is an assemb ly checkpoint for the posttranscriptional regulation of this flagellin gene. Deletion analysis within the 5' untranslated region of fljK ide ntified a sequence between +24 and +38 required for regulation of the fljK-lacZ fusion by class III genes, which implicates an imperfect 14- bp direct repeat in the posttranscriptional regulation of fljK. Our re sults show that fljL is also regulated posttranscriptionally by class III and unclassified flagellar genes, apparently by a mechanism differ ent from the one regulating fljK.