Analysis of the transcriptional pattern of the putative conjugation genes in Rickettsia bellii RML 369-C, an endosymbiont of Dermacentor variabilis

Rickettsia bellii is an obligate intracellular bacterium that was isolated from Dermacentor variabilis at the NIAID Rocky Mountain Laboratories. Rickettsia bellii encodes a set of conjugative transfer (tra) genes putatively involved in bacterial conjugation, however, their biological significance in nature remains unknown. We characterized the dynamics of R. bellii tra genes in comparison to genes transcribed stably and above the background level to understand when and at what levels the tra genes were active. traA_Ti was up-regulated at 72 hours post inoculation in the tick cell line ISE6, but showed no apparent changes in mammalian Vero and L929 cells. The transcription of the tra genes was positively correlated with one another and up-regulated from 12 to 72 hours post inoculation when compared to RBE_0422 (an inactivated transposase-derivative found within the tra cluster). Thus, 72 hours post infection may be the best time to observe R. bellii for signs of tra function, especially for evaluating its potential function in bacterial conjugation in a tick cell-derived culture system.