Background During syphilis infection, the envelope of Treponema pallidum is constantly exposed to the host environment and, therefore, the most likely target of the host defences against the invading spirochaete. The mechanisms that maintain T pallidum envelope integrity and functionality, particularly in response to host-induced stresses, are however poorly understood, and their elucidation would likely help identify important pathogenesis-associated molecules, perhaps related to T pallidum's ability to persist in the host despite a robust immune response. We hypothesised that in T pallidum, similarly to other Gram-negative pathogens, the transcription factor σ24 (σ24, encoded by the rpoE gene, TP0092) might be a key element in maintaining T pallidum envelope homeostasis. Putative σ24 binding motifs can be identified in silico upstream of several T pallidum genes that were experimentally shown to be involved in envelope stress response (ESR) in Escherichia coli. Furthermore, during early experimental syphilis σ24 is highly transcribed compared to other σ factors, and its expression increases even more as primary infection progresses. We therefore decided to investigate the possible role of σ24 in T pallidum ESR by identifying the components of the T pallidum σ24 regulon.
Methods T pallidum cells grown in rabbits were fixed after harvest to crosslink DNA-binding proteins to their target sequences in the chromosome. DNA sequences recognised by σ24 in vivo were isolated using chromatin immunoprecipitation in combination with high-throughput DNA sequencing (ChIP-seq) to identify bound DNA regions.
Results Thirty-nine DNA fragments targeted by σ24 were identified in the T pallidum chromosome. Seven of these target genes (lon-1, greA, ftsZ, prfB, htrA, and rpoE) were previously reported to be induced in response to envelope stress in E coli, suggesting that the T pallidum σ24 regulon is likely to be similar to that of other bacteria. Other putative target genes encode transporters, cell division proteins and a subset of motility and chemotaxis proteins.
Conclusions In T pallidum, σ24 seems to control genes involved in a variety of cellular processes, including maintenance of envelope homeostasis and barrier function. Additional putative σ24-dependent functions, apparently not directly involved in ESR, could as well be important in helping T pallidum adapt to the host environment during the infection.