Redox regulation of the genes for cobinamide biosynthesis in Salmonella typhimurium

Abstract

Transcription of the cobinamide biosynthetic genes (the CobI operon) was induced under three different physiological conditions: anaerobiosis (anaerobic respiration or fermentation), aerobic respiration at low oxygen levels, and aerobic respiration with a partial block of the electron transport chain. After a shift to inducing conditions, there was a time lag of approximately 50 min before the onset of CobI induction. Under conditions of anaerobic respiration, the level of CobI transcription was dependent on the nature of both the electron donor (carbon and energy source) and the acceptor. Cells grown with electron acceptors with a lower midpoint potential showed higher CobI expression levels. The highest level of CobI transcription observed was obtained with glycerol as the carbon source and fumarate as the electron acceptor. The high induction seen with glycerol was reduced by mutational blocks in the glycerol catabolic pathway, suggesting that glycerol does not serve as a gratuitous inducer but must be metabolized to stimulate CobI transcription. In the presence of oxygen, CobI operon expression was induced 6-to 20-fold by the following: inhibition of cytochrome o oxidase with cyanide, mutational blockage of ubiquinone biosynthesis, and starvation of mutant cells for heme. We suggest that the CobI operon is induced in response to a reducing environment within the cell and not by the absence of oxygen per se.The B12 biosynthetic genes are organized into three functionally related gene clusters located at 41 min on the chromosome (19,20); all genes are transcribed counterclockwise and appear to comprise three operons. The CobI operon encodes cobinamide biosynthetic functions, the CoblI operon encodes dimethyl benzimidazole biosynthesis, and the CobIII operon encodes functions required to join cobinamide and dimethyl benzimidazole, forming cobalamin. Most B12 biosynthetic genes appear to be included in these three operons. It was established by Jeter et al. (19) that de novo biosynthesis of vitamin B12 in Salmonella typhimurium occurs only under anaerobic growth conditions. Under aerobic conditions, S. typhimurium can make B12 if provided with cobinamide, suggesting that only synthesis and/or decoration of the corrinoid ring (CobI operon functions) are absent aerobically. Cells can synthesize dimethyl benzimidazole and assemble B12 from cobinamide and dimethyl benzimidazole (CoblI and CoblII operon functions, respectively) under both aerobic and anaerobic conditions. Only four functions in S. typhimurium are known to require cobalamin (8,14,33,34,40; R. M. Jeter, submitted for publication).Escalante-Semerena and Roth (13) used Lac operon fusions to examine transcriptional regulation of the cobalamin biosynthetic genes. Their studies revealed that transcription of the cobinamide biosynthesis genes (CobI operon) is stimulated by cyclic AMP (cAMP) and that the end product (cobalamin or a derivative) signals repression. Furthermore, CobI transcription is strongly repressed in the presence of oxygen, suggesting the e...

A single regulatory gene integrates control of vitamin B12 synthesis and propanediol degradation

“…Although glycerol did not mediate aerobic autoinduction of pocR transcription, anaerobic expression of a pocR-lac fusion was maximal during growth on glycerol-fumarate, suggesting that anaerobic metabolism of glycerol resulted in strong induction of pocR gene transcription. This effect is independent of the PocR function and may be mediated by a global redox control system as suggested previously (3).…”

“…3 is drawn) with an inferred protein product that is highly homologous to the araC family of transcriptional regulatory proteins (10,30 Previous studies indicated that transcription of the cob operon was induced only in the absence of oxygen (17,24). Subsequently, evidence was presented that a highly reduced cell interior (rather than absence of oxygen per se) was needed for induction (3). In (18,25).…”

“…In S. typhimurium, de novo vitamin B12 synthesis occurs only under anaerobic growth conditions; however, exogenous corrinoids can be taken up and converted to vitamin B12 coenzymes both aerobically and anaerobically (18). Previous work suggested that the cob operon is induced only anaerobically (redox control) and is stimulated by the complex of the cyclic AMP receptor protein (CRP) with cyclic AMP (cAMP) (3,17). Rondon and Escalante-Semerena recently found that the cob operon can also be induced under aerobic conditions by propanediol (29).…”

The control region of the pdu/cob regulon in Salmonella typhimurium

“…Thus, the expression of the normal cob operon in cells grown aerobically on glucose is minimal. Regulatory mutants that express the cob operon at high levels under these conditions were isolated (4,5). Two of these mutations, cobR4 and cobR58, are described here.…”

The ability of Salmonella to enter mammalian cells is affected by bacterial growth state.

“…The actual regulatory signal might be the concentration of molecular oxygen or another physiological property that correlates with the growth states that we have identified. For example, the anaerobic induction of cobinamide biosynthesis in S. typhimurium appears to be a result of a partial block in electron transport rather than the lack of oxygen per se (37). Once Salmonella invasion factors are identified, it would be instructive to examine their expression in bacteria located in different sites of infected animals.…”