Genetic Approaches to the Synthesis and Physiological Significance of B12 in Salmonella typhimurium
In the bacterium Salmonella typhimurium the biosynthetic genes for cobalamin are located in three separated regions of the genetic map. Most synthetic genes (approximately 20 of 25) are located in a single large operon whose expression is stimulated by cAMP and by a reduced cell interior; this operon is repressed by adenosyl-B12. Nutritional requirements of mutants suggest that an early intermediate in corrinoid biosynthesis (prior to cobyric acid in the pathway) is adenosylated and that adenosyl cobinamide is an obligatory precursor to completion of cobalamin synthesis (addition of dimethyl benzimidazole). Only one gene has been associated with the ability to perform adenosylation in the de novoe pathway; this gene is one of several alternatives needed to adenosylate exogenouse corrinoids. One gene has been associated with ability to synthesize aminopropanol.
Salmonella uses about 1% of its genome to encode the synthesis and transport of B12. While this large genetic investment suggests an important physiological role for the cofactor, the four known B12-dependent functions do not appear to be essential for growth of wild type Salmonella under the laboratory conditions tested. It is particularly puzzling that Salmonella only synthesizes B12 when growing anaerobically.
KeywordsAnaerobic Condition Methionine Synthesis Operon Expression Main Promoter Homocysteine Methyl Transferase
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