σ28 RNA polymerase regulates hctB, a late developmental gene in Chlamydia

Chlamydia is predicted to encode two alternative sigma factors that could provide a mechanism for the regulation of gene expression via alternative forms of RNA polymerase. We have demonstrated that sigma(28), one of these alternative sigma factors, is transcriptionally active. Chlamydial sigma(28) RNA polymerase was reconstituted from recombinant sigma(28) protein and core enzyme that was biochemically isolated from chlamydiae. In an in vitro transcription assay, sigma(28) RNA polymerase transcribed the hctB promoter in a sigma(28)-dependent manner. Transcription by sigma(28) RNA polymerase was salt tolerant compared with transcription by sigma(66) RNA polymerase, the major form of chlamydial RNA polymerase. As hctB encodes a histone-like protein that is only expressed late in the developmental cycle, our results suggest that sigma(28) RNA polymerase has a role in the regulation of late gene expression in Chlamydia.