| Popis: |
A light-inducible promoter, P B , drives expression of the carB operon in Myxococcus xanthus. Repressed by CarA in the dark, P B 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 P B . Intriguingly, P B 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 P B 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 . |
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