Induction of a Group 2 σ Factor, RPOD3, by High Light and the Underlying Mechanism in Synechococcus elongatus PCC 7942

• Published in: The Journal of biological chemistry Vol. 282; no. 51; p. 36887
• Main Authors: Asako Seki, Mitsumasa Hanaoka, Yuki Akimoto, Susumu Masuda, Hideo Iwasaki, Kan Tanaka
• Format: Journal Article
• Language: English
• Published: American Society for Biochemistry and Molecular Biology 21-12-2007
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M707582200

Among the σ 70 family bacterial σ factors, group 2 σ factors have similar promoter recognition specificity to group 1 (principal) σ factors
and express and function under specific environmental and physiological conditions. In general, the cyanobacterial genome
encodes more than four group 2 σ factors, and the unicellular Synechococcus elongatus PCC 7942 ( Synechococcus ) has five group 2 σ factors (RpoD2–6). In this study, we analyzed expression of group 2 σ factors of Synechococcus at both mRNA and protein levels, and we showed that the rpoD3 expression was activated only by high light (1,500 μmol photons m –2 s –1 ) among the various stress conditions examined. After high light shift, rpoD3 mRNA accumulated transiently within the first 5 min and diminished subsequently, whereas RpoD3 protein increased gradually
during the first several hours. We also found that the rpoD3 deletion mutant rapidly lost viability under the same conditions. Analysis of the rpoD3 promoter structure revealed the presence of an HLR1 ( h igh l ight- r esponsive element 1 ) sequence, which was suggested to be responsible for the high light-induced transcription under the control of the NblS (histidine
kinase)-RpaB (response regulator) two-component system (Kappell, A. D., and van Waasbergen, L. G. (2007) Arch. Microbiol. 187, 337–342), at +6 to +23 with respect to the transcriptional start site. Here we demonstrated that recombinant RpaB protein
specifically bound to HLR1 of the rpoD3 and hliA genes in vitro , and overexpression of a truncated RpaB variant harboring only the phosphoreceiver domain derepressed the transcription in vivo . Thus, we have concluded that phosphorylated RpaB are repressing the rpoD3 and hliA transcription under normal growth conditions, and the RpaB dephosphorylation induced by high light stress results in transcriptional
derepression.