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Published on: July 21, 2014
The SOS response and functional diversification of the transcription factor LexA in cyanobacteria
Haruka Kubodera1, Hiroki Inoue2, Aoi Ando1
1Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Saitama 338-8570, Japan.
Abstract:
The SOS response is a widespread bacterial mechanism for coping with DNA-damaging stress. The key regulator of the process is the transcriptional repressor LexA, which represses expression of DNA-repair-related genes under nonstress conditions and undergoes self-cleavage to induce them under DNA-damaging conditions. Here, we gained insights into the SOS response and functional diversification of LexA in cyanobacteria by characterizing 2 model species, Synechocystis sp. PCC 6803 (S.6803) and Anabaena sp. PCC 7120 (A.7120), together with an early branching species Gloeobacter violaceus PCC 7421 (G.7421) whose LexA belongs to the same subclade as those of Candidatus Melainabacteria. After 3 h of treatment with UV-C or UV-B, the amount of LexA in A.7120 and G.7421 decreased to 30% to 40% of pre-stress levels concomitant with the induction of DNA repair-related genes, suggesting that LexA may function as a typical SOS repressor. In S.6803, on the other hand, we did not observe a decrease in LexA after treatment with UV radiation. This observation was consistent with previous studies reporting that LexA with substituted self-cleavage residues regulates expression of genes related to various cellular functions but not the SOS response in S.6803. The changes in gene expression profile of S.6803 after exposure to UV-C seemed to depend on transcription factors other than LexA, including the master regulator of photosynthetic genes, RpaB. In phototrophic cyanobacteria, the role of LexA as an SOS repressor may have become less important over the course of evolution, enabling LexA to diversify its functions in some species.
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