Motility Characteristics Are Altered for Rickettsia bellii Transformed To Overexpress a Heterologous rickA Gene

The rickettsial protein RickA activates host cell factors associated with the eukaryotic actin cytoskeleton and is likely involved with rickettsial host cell binding and infection and the actin-based motility of spotted fever group rickettsiae. TherickA gene sequence and protein vary substantially betweenRickettsia species, as do observed motility-associated phenotypes. To help elucidate the function of RickA and determine the effects of species-specific RickA variations, we compared extracellular binding, intracellular motility, and intercellular spread phenotypes of threeRickettsia bellii variants. These included two shuttle vector-transformedR. bellii strains and the wild-type isolate from which they were derived,R. bellii RML 369C. Both plasmid shuttle vectors carried spectinomycin resistance and a GFPuv reporter; one containedRickettsia monacensis -derivedrickA , and the other lacked therickA gene.Rickettsia bellii transformed to expressR. monacensis rickA highly overexpressed this transcript in comparison to its nativerickA . These rickettsiae also moved at higher velocities and followed a more curved path than the negative-control transformants. A lower proportion ofR. monacensis rickA -expressing bacteria ever became motile, however, and they formed smaller plaques.