Chlamydia Gene Regulation
Christopher J. Rosario, Katelyn Soules, P. Scott Hefty and Ming Tan
from: Chlamydia Biology: From Genome to Disease (Edited by: Ming Tan, Johannes H. Hegemann and Christine Sütterlin). Caister Academic Press, U.K. (2020) Pages: 219-240.
A defining feature of the Chlamydia developmental cycle is the expression of chlamydial genes in three main temporal classes. Early genes are the only genes transcribed at the beginning of the intracellular infection, but the mechanism for their selective expression is unknown. The large majority of chlamydial genes are midcycle genes, which are transcribed during the midstage of the developmental cycle at the time of reticulate body (RB) growth and replication. Gene expression in midcycle is proposed to be activated by higher DNA supercoiling levels and to be regulated by a DNA gyrase and two DNA topoisomerases, which are enzymes that control DNA supercoiling. Late genes are up-regulated late in the developmental cycle, when RB to elementary body (EB) conversion takes place. There are two subsets of late genes that are respectively transcribed by σ66RNA polymerase, which is the major chlamydial RNA polymerase, or an alternative RNA polymerase called σ28RNA polymerase. However, both subsets of late genes are repressed by the transcription factor EUO, which is thus known as the master regulator of late gene expression. Transcription is then proposed to be silenced in EBs by two histone-like proteins, Hc1 and Hc2, which condense DNA. Chlamydia gene expression is also controlled by transcription factors that homeostatically regulate the levels of amino acids, nucleotides, metal ions and the major heat shock proteins. Regulators of chlamydial gene expression with less understood roles include a second alternative sigma factor, σ54 , putative regulators of each of the three forms of chlamydial RNA polymerase, and the transcription factors, ChxR and GrgA. In addition, there are potentially many sRNAs that control gene expression by binding to target transcripts. Collectively, these mechanisms allow Chlamydia to regulate the expression of its genes in both a pre-programmed manner and in response to external signals read more ...