title: MarR-type transcriptional regulator ChlR activates expression of tetrapyrrole biosynthesis genes in response to low-oxygen conditions in cyanobacteria.
abstract: Oxygen is required for three enzyme reactions in chlorophyll and bilin biosynthesis pathways: coproporphyrinogen III oxidase (HemF), heme oxygenase (HO1), and Mg-protoporphyrin IX monomethylester cyclase (ChlA(I)). The cyanobacterium Synechocystis sp. PCC 6803 has alternative enzymes, HemN, HO2, and ChlA(II), to supply chlorophyll/bilins even under low-oxygen environments. The three genes form an operon, chlA(II)-ho2-hemN, that is induced in response to low-oxygen conditions to bypass the oxygen-dependent reactions. Here we identified a transcriptional regulator for the induction of the operon in response to low-oxygen conditions. A pseudorevertant, Δho1R, was isolated from a HO1-lacking mutant Δho1 that is lethal under aerobic conditions. Δho1R grew well even under aerobic conditions. In Δho1R, HO2 that is induced only under low-oxygen conditions was anomalously expressed under aerobic conditions to complement the loss of HO1. A G-to-C transversion in sll1512 causing the amino acid change from aspartate 35 to histidine was identified as the relevant mutation by resequencing of the Δho1R genome. Sll1512 is a MarR-type transcriptional regulator. An sll1512-lacking mutant grew poorly under low-oxygen conditions with a remarked decrease in Chl content that would be caused by the suppressed induction of the chlA(II) and hemN genes in Chl biosynthesis under low-oxygen conditions. These results demonstrated that Sll1512 is an activator in response to low-oxygen environments and that the D35H variant becomes a constitutive activator. This hypothesis was supported by a gel shift assay showing that the Sll1512-D35H variant binds to the DNA fragment upstream of the operon. We propose to name sll1512 chlR.
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan.
authors.forename: Rina
authors.initials: R
authors.lastname: Aoki
authors.forename: Tomoya
authors.initials: T
authors.lastname: Takeda
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Omata
authors.forename: Kunio
authors.initials: K
authors.lastname: Ihara
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Fujita
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D061146:Chlorophyll Binding Proteins / Q000235:genetics; D003304:Coproporphyrinogen Oxidase / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D005798:Genes, Bacterial / Q000502:physiology; D016680:Genome, Bacterial; D010100:Oxygen / Q000378:metabolism* / Q000494:pharmacology; D010641:Phenotype; D046939:Synechocystis / Q000201:enzymology* / Q000235:genetics*; D045725:Tetrapyrroles / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic / Q000502:physiology
chemical_list: D001426:Bacterial Proteins; D061146:Chlorophyll Binding Proteins; D003304:Coproporphyrinogen Oxidase; D004268:DNA-Binding Proteins; D010100:Oxygen; D045725:Tetrapyrroles; D014157:Transcription Factors; C109194:HemN protein, Bacteria