COBU-DEPENDENT ASSIMILATION OF NONADENOSYLATED COBINAMIDE IN COBA MUTANTS OF SALMONELLA-TYPHIMURIUM

The cobA locus of Salmonella typhimurium is involved in the assimilati on of nonadenosylated cobinamide, (CN)2CBI, into cobalamin (CBL) under aerobic and anaerobic growth conditions. Aerobically, cobA mutants ar e unable to assimilate (CN)2CBI into CBL. However, under anaerobic con ditions, cobA mutants assimilate (CN)2CBI into CBL as efficiently as c obA+ strains. On the basis of this observation, we postulated the exis tence of a cobA-independent pathway for the assimilation of (CN)2CBI i nto CBL that is functional under anaerobic growth conditions (J. C. Es calante-Semerena, S.-J. Suh, and J. R. Roth, J. Bacteriol. 172:273-280 , 1990). In this paper, we report the isolation and initial genetic ch aracterization of derivatives of cobA mutants that are unable to assim ilate (CN)2CBI into CBL during, anaerobic growth. As demonstrated by c omplementation analysis, marker rescue, and DNA sequencing data, these mutations are alleles of cobU, a gene involved in the assembly of the nucleotide loop of CBL. We have shown that the block in CBL synthesis in these cobU cobA double mutant strains can be corrected by exogenou s adenosyl-CBI. Our data indicate that this new class of cobU mutation s blocks CBL biosynthesis but does not destroy the putative kinase-gua nylyltransferase activities of the CobU protein. We propose that this new class of cobU mutations may affect an as yet unidentified ATP:corr inoid adenosyltransferase activity of the CobU protein. Alternatively, such mutations may alter the ability of CobU to use nonadenosylated C BI as a substrate.