As a first step towards the elucidation of the molecular mechanisms responsible for the utilization of choline and glycine betaine (betaine) either as carbon and nitrogen sources or as osmoprotectants in we selected a Tn5 mutant, LTS23-1020, which failed to grow on choline but grew on betaine. The mutant was deficient in choline dehydrogenase (CDH) activity, failed to oxidize [-C]choline to [-C]betaine, and did not use choline, but still used betaine, as an osmoprotectant. The Tn5 mutation in LTS23-1020 was complemented by plasmid pCH034, isolated from a genomic bank of 102F34. Subcloning and DNA sequencing showed that pCH034 harbours two ORFs which showed 60% and 57% identity with the gene encoding betaine-aldehyde dehydrogenase (BADH) and gene encoding CDH, respectively. In addition to the homology with genes, the deduced sequence of the sinorhizobial BADH protein displays consensus sequences also found in plant BADHs. The deduced sequence of the sinorhizobial CDH protein shares only 21% identical residues with choline oxidase from The structural organization of the genes in differs from that described in (i) the two ORFs are separated by a 210 bp sequence containing inverted repeats ressembling a putative rho-independent transcription terminator, and (ii) no sequence homologous to (high-affinity choline transport system) or (regulator) was found in the vicinity of the sinorhizobial genes. Evidence is also presented that the genes are not located on the megaplasmids.


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