THE FLGK MOTILITY OPERON OF BORRELIA-BURGDORFERI IS INITIATED BY A SIGMA(70)-LIKE PROMOTER

A duster of flagellar genes of Borrelia burgdorferi was identified and sequenced. This cluster comprises an operon, designated the flgK oper on, which is initiated by a sigma(70)-like promoter. The flgK operon c onsists of flbF (function unknown), flgK (encoding HAP1), flgL (encodi ng HAP3) and orfX (function unknown), and maps at 185 kb on the chromo some. In other bacteria, the hook-associated proteins HAP1 and HAP3 co nnect the flagellar filament to the hook and are required for the last stage of flagellar assembly. Reverse transcriptase-PCR analysis indic ated that flbF through to orfX are transcribed as a single mRNA, and p rimer extension analysis revealed that transcription of the flgK opero n is initiated by a sigma(70)-like promoter upstream of flbF. Subcloni ng the flgK promoter element into a promoter probe cat vector revealed that the flgK promoter element had strong activity in both Escherichi a coli and Salmonella typhimurium. In addition, when this construct wa s transformed into a fliA mutant of 5, typhimurium which lacked a func tional flagellar-specific sigma(28) factor, the flgK promoter was stil l functional. Based on these results, the promoter element of the flag ellin gene (fla, hereafter referred to as flag) was re-examined, flag encodes the flagellar filament protein, and a sigma(gp33-34)-like prom oter has been reported to be involved in the transcription of this gen e. A transcriptional start point was found 1 bp downstream of the repo rted start site. The sequence around -10 and -35 are consistent with t he presence of a sigma(70)-like promoter in addition to the putative s igma(gp33-34)-like promoter for flag. In contrast to the flgK promoter element, no activity was detected after subcloning a flaB promoter el ement into the promoter probe cat vector. Because a sigma(70)-like pro moter rather than a unique flagellar sigma factor is involved in the l ater stage of flagellar assembly, the regulation of B. burgdorferi fla gellar genes is evidently different from that of other bacteria.