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Light Sensing with Vitamin B12

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Sci. Signal.  10 May 2011:
Vol. 4, Issue 172, pp. ec131
DOI: 10.1126/scisignal.4172ec131

Whereas a role for vitamin A in light-mediated signaling is well known (indeed this is responsible for human vision), less is known about a role for vitamin B12 in light-mediated signaling. Following on previous work showing that cobalamin (B12) is required for repression of a light-inducible promoter in Myxococcus xanthus, Ortiz-Guerrero et al. identified a molecular mechanism for light-induced alleviation of the repressive activity of CarH, which was mediated by B12. CarH and CarA are structurally similar repressors of the carotenogenic genes that protect bacteria from photooxidative damage, but only CarH requires B12 for its function. These carotenogenic genes are repressed in the dark and stimulated in the light. Replacing the C-terminal domain of CarA with that of CarH conferred B12-dependent repressor activity to CarA. To produce sufficient protein for biochemical analysis, the authors constructed chimeric proteins of the N-terminal (DNA-binding) domain of CarA or CarR fused to the C-terminal domain of Thermus thermophilus homolog of CarH (CTt1 or CTt2, respectively). Whereas the binding of CarA to the CarA/CarH operator was not affected by the presence of any form of B12, CTt1 and CTt2 both exhibited increased DNA binding in the presence of AdoB12 but not in the presence of MeB12 or CNB12. Knocking out the only gene in M. xanthus that can convert inactive CNB12 to active AdoB12 resulted in loss of repression of carotenogenic genes in a strain engineered to depend solely on CarH for repression. Two-hybrid analysis (performed in the dark) showed that the C-terminal domains of CarH or T. thermophilus CarH (TtCarHCt) interacted only in the presence of B12, and size exclusion chromatography (performed in the dark) showed that in the presence of AdoB12 the TtCarHCt formed tetramers, whereas in its absence this fragment was monomeric. Light at wavelengths that trigger the cleavage of a bond in AdoB12 (ultraviolet, blue, or green light, but not red light, which AdoB12 does not absorb) impaired the interaction of the C-terminal domains in the two-hybrid system, caused these protein domains to migrate as monomers in size exclusion chromatography experiments, and reduced the DNA-binding affinity of CTt1 and CTt2. In an M. xanthus strain solely dependent on CarH for carotenogenic gene repression, white light, but not red light, stimulated the conversion of the bacteria from yellow to red, consistent with induction of the carotenogenic genes, and induction of a CarA- or CarH-responsive reporter required light and the presence of B12. Thus, this work suggests that B12 functions as a light sensor in bacteria to control the oligomerization and DNA-binding affinity of a transcriptional regulator.

J. M. Ortiz-Guerrero, M. C. Polanco, F. J. Murillo, S. Padmanabhan, M. Elías-Arnanz, Light-dependent gene regulation by a coenzyme B12-based photoreceptor. Proc. Natl. Acad. Sci. U.S.A. 108, 7565–7570 (2011). [Abstract] [Full Text]

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