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510051 
Journal Article 
Vitamin B-12 partners the CarH repressor to downregulate a photoinducible promoter in Myxococcus xanthus 
Perez-Marin, MC; Padmanabhan, S; Polanco, MC; Murillo, FJ; Elias-Arnanz, M 
2008 
1 
Molecular Microbiology
ISSN: 0950-382X 
67 
4 
804-819 
English 
A light-inducible promoter, P-B, drives expression of the carB operon in Myxococcus xanthus. Repressed by CarA in the dark, PB is activated when CarS, produced in the light, sequesters CarA to prevent operator-CarA binding. The MerR-type, N-terminal domain of CarA, which mediates interactions with both operator and CarS, is linked to a C-terminal oligomerization module with a predicted cobalamin-binding motif. Here, we show that although CarA does bind vitamin B-12, mutating the motif involved has no effect on its ability to repress PB. Intriguingly, PB could be repressed in the dark even with no CarA, so long as B-12 and an intact CarA operator were present. We have discovered that this effect of B-12 depends on the gene immediately downstream of carA. Its product, CarH, also consists of a MerR-type, N-terminal domain that specifically recognizes the CarA operator and CarS, linked to a predicted B-12-binding C-terminal oligomerization module. The B-12-mediated repression of PB in the dark is relieved by deleting carH, by mutating the DNA- or B-12-binding residues of CarH, or by illumination. Our findings unveil parallel regulatory circuits that control a light-inducible promoter using a transcriptional factor repertoire that includes a paralogous gene pair and vitamin B-12. 
light-induced carotenogenesis; dependent methionine synthase; non-phototrophic bacteria; escherichia-coli; carotenoid synthesis; gene-expression; blue-light; salmonella-typhimurium; b-12-dependent; enzymes; transcriptional factor