1. J Bacteriol. 2006 Sep;188(18):6661-8.

Thiamine is synthesized by a salvage pathway in Rhizobium leguminosarum bv.
viciae strain 3841.

Karunakaran R, Ebert K, Harvey S, Leonard ME, Ramachandran V, Poole PS.

School of Biological Sciences, University of Reading, Whiteknights, P.O. Box 228,
Reading RG6 6AJ, United Kingdom.

In the absence of added thiamine, Rhizobium leguminosarum bv. viciae strain 3841 
does not grow in liquid medium and forms only "pin" colonies on agar plates,
which contrasts with the good growth of Sinorhizobium meliloti 1021,
Mesorhizobium loti 303099, and Rhizobium etli CFN42. These last three organisms
have thiCOGE genes, which are essential for de novo thiamine synthesis. While R. 
leguminosarum bv. viciae 3841 lacks thiCOGE, it does have thiMED. Mutation of
thiM prevented formation of pin colonies on agar plates lacking added thiamine,
suggesting thiamine intermediates are normally present. The putative functions of
ThiM, ThiE, and ThiD are 4-methyl-5-(beta-hydroxyethyl) thiazole (THZ) kinase,
thiamine phosphate pyrophosphorylase, and 4-amino-5-hydroxymethyl-2-methyl
pyrimidine (HMP) kinase, respectively. This suggests that a salvage pathway
operates in R. leguminosarum, and addition of HMP and THZ enabled growth at the
same rate as that enabled by thiamine in strain 3841 but elicited no growth in
the thiM mutant (RU2459). There is a putative thi box sequence immediately
upstream of the thiM, and a gfp-mut3.1 fusion to it revealed the presence of a
promoter that is strongly repressed by thiamine. Using fluorescent microscopy and
quantitative reverse transcription-PCR, it was shown that thiM is expressed in
the rhizosphere of vetch and pea plants, indicating limitation for thiamine. Pea 
plants infected by RU2459 were not impaired in nodulation or nitrogen fixation.
However, colonization of the pea rhizosphere by the thiM mutant was impaired
relative to that of the wild type. Overall, the results show that a thiamine
salvage pathway operates to enable growth of Rhizobium leguminosarum in the
rhizosphere, allowing its survival when thiamine is limiting.

PMCID: PMC1595474
PMID: 16952958 [PubMed - indexed for MEDLINE]