Cloning of the fliI gene from Rhodobacter sphaeroides WS8 by analysis of a transposon mutant with impaired motility.

A transposon mutant of Rhodobacter sphaeroides WS8 was isolated that showed reduced swarming on soft agar plates. Liquid cultures of this mutant (M18) showed a low percentage of motile swimming cells in mid-exponential phase and a low level of extracellular flagellin protein by Western blotting. M18 was complemented by a clone from a library of R. sphaeroides WS8 DNA, and restriction mapping of the site of TnphoA insertion in the mutant, coupled with DNA sequencing, showed that it had a defect in the fliI gene. To determine if a partly functional fliI gene was giving the low-motility phenotype of M18, a drug resistance omega cartridge was inserted into the gene to give a complete null mutant. This null strain also produced a low percentage of motile cells. Possible reasons for this apparent fliI-independent flagellar formation are discussed.