STRUCTURAL AND GENETIC-ANALYSIS OF A MUTANT OF RHODOBACTER-SPHAEROIDES WS8 DEFICIENT IN HOOK LENGTH CONTROL

Motility in the photosynthetic bacterium Rhodobacter sphaeroides is ac hieved by the unidirectional rotation of a single subpolar flagellum. In this study, transposon mutagenesis was used to obtain nonmotile fla gellar mutants from this bacterium. We report here the isolation and c haracterization of a mutant that shows a polyhook phenotype. Morpholog ical characterization of the mutant was done by electron microscopy. P olyhooks were obtained by shearing and were used to purify the hook pr otein monomer (FIgE). The apparent molecular mass of the hook protein was 50 kDa. N-terminal amino acid sequencing and comparisons with the hook proteins of other flagellated bacteria indicated that the Rhodoba cter hook protein has consensus sequences common to axial flagellar co mponents. A 25-kb fragment from an R. sphaeroides WS8 cosmid library r estored wild-type flagellation and motility to the mutant. Using DNA a djacent to the inserted transposon as a probe, we identified a 4.6-kb SalI restriction fragment that contained the gene responsible for the polyhook phenotype. Nucleotide sequence analysis of this region reveal ed an open reading frame with a deduced amino acid sequence that was 2 3.4% identical to that of FliK of Salmonella typhimurium, the polypept ide responsible for hook length control in that enteric bacterium. The relevance of a gene homologous to fliK in the uniflagellated bacteriu m R. sphaeroides is discussed.