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The rpsD gene of Chlamydia trachomatis encodes the alternative σ factor σ 28, which bears strong homology to many bacterial σ factors, including Escherichia coli σ 28 and Bacillus subtilis σ B and σ D. Recently, a σ 28 promoter was identified upstream of the late-cycle-expressed gene hctB, which encodes the Chlamydia-histone-like protein 2 ( Yu & Tan, 2003 ). In this study it is shown that the product of chlamydial rpsD is an E. coli σ 28 homologue. It was found that recombinant chlamydial σ 28, in combination with E. coli core RNA polymerase, initiates transcription in vitro from the E. coli σ 28-dependent promoter of fliC. It was also demonstrated that the recombinant chlamydial σ 28 does not recognize major σ factor σ 70-consensus-like sequences in vitro. In C. trachomatis-infected cells, two rpsD transcripts were detected with 5′ ends located 18 (transcript I) and 54 bp (transcript II) upstream of the translational initiation codon at 16 and 30 h post-infection. When the temperature of cultures infected with C. trachomatis was shifted from 35 to 42 °C, the rpsD transcript I increased dramatically. The levels of chlamydial σ 28, relative to EF-Tu, were greater throughout the exponential growth phase of the reticulate body, but lower late in the developmental cycle. These data support the hypothesis that σ 28 plays a role in the regulatory network that allows chlamydiae to survive changes in its environment, enabling it to complete its unique developmental cycle.
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