ABSTRACTChlamydiais a genus of pathogenic bacteria with an unusual intracellular developmental cycle marked by temporal waves of gene expression. The three main temporal groups of chlamydial genes are proposed to be controlled by separate mechanisms of transcriptional regulation. However, we have noted genes with discrepancies, such as the early genednaKand the midcycle genesbioYandpgk, which have promoters controlled by the late transcriptional regulators EUO and σ28. To resolve this issue, we analyzed the promoters of these three genesin vitroand inChlamydia trachomatisbacteria grown in cell culture. Transcripts from the σ28-dependent promoter of each gene were detected only at late times in the intracellular infection, bolstering the role of σ28RNA polymerase in late gene expression. In each case, however, expression prior to late times was due to a second promoter that was transcribed by σ66RNA polymerase, which is the major form of chlamydial polymerase. These results demonstrate that chlamydial genes can be transcribed from tandem promoters with different temporal profiles, leading to a composite expression pattern that differs from the expression profile of a single promoter. In addition, tandem promoters allow a gene to be regulated by multiple mechanisms of transcriptional regulation, such as DNA supercoiling or late regulation by EUO and σ28. We discuss how tandem promoters broaden the repertoire of temporal gene expression patterns in the chlamydial developmental cycle and can be used to fine-tune the expression of specific genes.IMPORTANCEChlamydiais a pathogenic bacterium that is responsible for the majority of infectious disease cases reported to the CDC each year. It causes an intracellular infection that is characterized by coordinated expression of chlamydial genes in temporal waves. Chlamydial transcription has been shown to be regulated by DNA supercoiling, alternative forms of RNA polymerase, and transcription factors, but the number of transcription factors found inChlamydiais far fewer than the number found in most bacteria. This report describes the use of tandem promoters that allow the temporal expression of a gene or operon to be controlled by more than one regulatory mechanism. This combinatorial strategy expands the range of expression patterns that are available to regulate chlamydial genes.
Altering gene expression by aminocoumarins: the role of DNA supercoiling in Staphylococcus aureus