Molecular mechanisms of chlamydial transcription regulation.

摘要

The intracellular pathogen Chlamydia employs a highly regulated transcriptional program to coordinate the expression of its genes. The temporal regulation of its genes into three expression classes corresponding to the early, mid, and late stages of its developmental cycle is a hallmark of chlamydial growth and replication. In addition, chlamydiae maintain homeostatic control of genes involved in the synthesis or import of essential nutrients such as amino acids, nucleotides and metal ions. In this dissertation, we investigated three mechanisms by which Chlamydia regulate transcription. First, we studied an alternative form of chlamydial RNA polymerase, sigma28 RNA polymerase, by using a comprehensive mutational approach to define the promoter sequence that it recognizes. This work has led to the identification of new sigma28 target genes, which has provided experimental support for sigma28 RNA polymerase as a regulator of late gene expression. Second, we studied the regulation of genes encoding the Type III Secretion (T3S) system and demonstrated a correlation between their response to DNA supercoiling and their temporal expression pattern. Specifically, we found that promoters of T3S mid genes, like other mid genes, are stimulated by increased negative DNA supercoiling, which supports a general role for DNA supercoiling in the up-regulation of chlamydial transcription during midcycle. Finally, by showing that C. trachomatis CT406 is the homolog of NrdR, and thus the repressor of the genes encoding ribonucleotide reductase, we have described a critical regulator in Chlamydia. Since ribonucleotide reductase catalyzes the conversion of NTPs into dNTPs, and this is the only source of dNTPs for chlamydiae, NrdR is likely to play a key role in the regulation of DNA replication. These studies of chlamydial transcriptional regulation demonstrate that this highly successful bacterial pathogen has developed diverse mechanisms to regulate its gene expression inside an infected cell.