Role for Vitamin B ₁₂ in Light Induction of Gene Expression in the Bacterium Myxococcus xanthus

Abstract

A light-inducible promoter (P B ) drives the carB operon (carotenoid genes) of the bacterium Myxococcus xanthus. A gene encoding a regulator of carotenoid biosynthesis was identified by studying mutant strains carrying a transcriptional fusion to P B and deletions in three candidate genes. Our results prove that the identified gene, named carA, codes for a repressor of the P B promoter in the dark. They also show that the carA gene product does not participate in the light activation of two other promoters connected with carotenoid synthesis or its regulation in M. xanthus. CarA is a novel protein consisting of a DNA-binding domain of the family of MerR helix-turn-helix transcriptional regulators, directly joined to a cobalamin-binding domain. In support of this, we report here that the presence of vitamin B 12 or some other cobalamin derivatives is absolutely required for activation of the P B promoter by light.Cells of the gram-negative bacterium Myxococcus xanthus respond to blue light by accumulating significant amounts of carotenoids. This response relies on the transcriptional activation of the structural genes for carotenoid synthesis (Fig. 1A) (see reference 17 for a review). All but one of the carotenoid biosynthesis genes are clustered together at the carB operon. The exception is gene crtI, unlinked to carB. Regulatory genes participating in the activation of the crtI promoter (P I ) by blue light have been identified. Two of them, carQ and carR, form part of the carQRS operon, the promoter of which (P QRS ) is also regulated by blue light. Protein CarQ is a member of the extracytoplasmic function (ECF) subfamily of factors. CarR, a membrane-spanning protein, acts as an anti-factor, sequestering protein CarQ to the membrane in the dark. Illumination of the cells somehow results in the loss of CarR, leaving CarQ free to activate the P QRS and P I promoters. These two promoters share two DNA segments, centered at the Ϫ31 and Ϫ10 positions, which correspond to the binding sites of CarQ. Another protein, CarD, which contains a DNA-binding domain similar to that of eukaryotic high-mobility group A proteins, is required independently for light activation of the P I and P QRS promoters. The latter also requires the normal action of integration host factor (10,12,23,27).Less is known on the regulation of the carB promoter (P B ), but previous lines of evidence indicated that the mechanism of the action of light is different from the one involved in the activation of P QRS and P I . On the one hand, the P B promoter lacks the binding sites for the ECF-factor CarQ, showing instead a Ϫ35 sequence that perfectly matches the Ϫ35 consensus for the major bacterial factor (6). On the other hand, the P B promoter, but not P QRS nor P I , is affected by mutations at two unlinked genes. One is carS, the third gene of the carQRS operon mentioned above. The first known carS mutation (carS1) was identified because it caused constitutive expression from the normally light-inducible P B promoter (4). Mutation carS1 is, howev...

Identification and in vivo characterization of PpaA, a regulator of photosystem formation in Rhodobacter sphaeroides

“…Interestingly, evidence from R. capsulatus suggests that shown to perform sensory functions, in addition to catalytic functions (Beinert & Kiley, 1999;Christie & Briggs, 2001;Gilles-Gonzalez, 2001;Braatch et al, 2002;Masuda & Bauer, 2002;. Two groups hypothesized that corrinoids might regulate activities of transcription factors (Roof & Roth, 1992;Sheppard & Roth, 1994;Cervantes & Murillo, 2002). The latter report is especially relevant because it considers a possibility that DNA-binding activity of M. xanthus ORF10 (CarA) might be corrinoiddependent.…”

“…8). ORF10 is a transcriptional regulator, whose DNA-binding motif resembles a well characterized, MerR-like, helix-turn-helix domain (Cervantes & Murillo, 2002). In contrast to ORF10, no DNA-binding motif could be identified in PpaA or its homologues.…”

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

“…S1B). Besides their autonomously stable, N-terminal DNA-binding domains (CarANt and CarHNt, respectively) that recognize the same operator (12,19,20), CarA and CarH have C-terminal domains (CarACt and CatHCt, respectively) with a B 12 -binding motif of the type found in enzymes like methionine synthase (6,12,(21)(22)(23). However, only CarH requires B 12 for its repressor activity (12).…”

New light on vitamin B12: The adenosylcobalamin-dependent photoreceptor protein CarH

“…20 The amino acid sequence of the carboxyl region of CarH is very similar to that of the MeCbl-binding domain of methionine synthase and shows a typical cobalamin-binding fingerprint sequence. 6,13,18,19,31,32 However, the prosthetic group of CarH is AdoCbl, rather than MeCbl, as in methionine synthase. 6 AdoCbl is bound to CarH through displacement of dbzm by the imidazole group of a conserved histidine (His193 in M. xanthus and His177 in T. thermophilus), similarly to the binding of MeCbl in methionine synthase.…”

“…6,[18][19][20][21][22][23][24][25][26][27] These bacteria produce carotenoids -yellow, orange or red pigments -which protect them from photo-oxidative damage by quenching the singlet oxygen free radicals produced by absorption of energy from light. 19 The photoreceptor proteins were first discovered in the bacterium Myxococcus xanthus, 6,[18][19][20] which turns red in the presence of light, as a result of the synthesis of carotenoids, but is pale yellow in the dark. However, carotenoids have also been found in a variety of other bacteria, including Streptomyces coelicolor [21][22][23] , Thermus thermophilus [24][25][26] and Bacillus megaterium 27 .…”