Vitamin B₁₂ partners the CarH repressor to downregulate a photoinducible promoter in Myxococcus xanthus

Abstract

SummaryA light-inducible promoter, PB, 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 cobalaminbinding motif. Here, we show that although CarA does bind vitamin B12, 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 B12 and an intact CarA operator were present. We have discovered that this effect of B12 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 B12-binding C-terminal oligomerization module. The B12-mediated repression of PB in the dark is relieved by deleting carH, by mutating the DNA-or B12-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 B12.

Light-dependent gene regulation by a coenzyme B 12 -based photoreceptor

“…Typically, B 12 functions as an essential cofactor of enzymes such as isomerases, methyltransferases, and reductases (6,7). B 12 has also been implicated in directly regulating gene expression via the B 12 riboswitch, an RNA-based mechanism widespread in bacteria (8)(9)(10)(11). Here, specific binding of AdoB 12 to the 5′-untranslated messenger RNA leader sequence of a target gene or operon (usually involved in B 12 synthesis or transport) allosterically modulates RNA structure to regulate gene expression.…”

“…B 12 has also been implicated in directly regulating gene expression via the B 12 riboswitch, an RNA-based mechanism widespread in bacteria (8)(9)(10)(11). Here, specific binding of AdoB 12 to the 5′-untranslated messenger RNA leader sequence of a target gene or operon (usually involved in B 12 synthesis or transport) allosterically modulates RNA structure to regulate gene expression. In contrast to this RNA-based switch, B 12 -directed gene regulation via a protein factor is virtually uncharted.…”

“…It is also the largest and most complex of nonpolymeric biomolecules, whose central feature is a tetrapyrrole-derived cobalt-containing corrin moiety. Two distinct B 12 forms are used in vivo (4,6): one (AdoB 12 ) has 5′-deoxyadenosine and the other (methylcobalamin, MeB 12 ) has a methyl coordinated to the corrin cobalt. The stable but nonfunctional vitamin B 12 or cyanocobalamin (CNB 12 ), with a cyano-cobalt bond, must be converted to AdoB 12 or MeB 12 for intracellular use.…”

“…Two distinct B 12 forms are used in vivo (4,6): one (AdoB 12 ) has 5′-deoxyadenosine and the other (methylcobalamin, MeB 12 ) has a methyl coordinated to the corrin cobalt. The stable but nonfunctional vitamin B 12 or cyanocobalamin (CNB 12 ), with a cyano-cobalt bond, must be converted to AdoB 12 or MeB 12 for intracellular use. Typically, B 12 functions as an essential cofactor of enzymes such as isomerases, methyltransferases, and reductases (6,7).…”

“…The stable but nonfunctional vitamin B 12 or cyanocobalamin (CNB 12 ), with a cyano-cobalt bond, must be converted to AdoB 12 or MeB 12 for intracellular use. Typically, B 12 functions as an essential cofactor of enzymes such as isomerases, methyltransferases, and reductases (6,7). B 12 has also been implicated in directly regulating gene expression via the B 12 riboswitch, an RNA-based mechanism widespread in bacteria (8)(9)(10)(11).…”