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J. Bacteriol., Jan 1995, 66-74, Vol 177, No. 1
SA Lacks, B Greenberg and P Lopez
Two genes, sulB and sulC, in a folate biosynthetic gene cluster of
Streptococcus pneumoniae were identified after determination of the DNA
sequence between two previously reported genes, sulA and sulD, in a cloned
segment of chromosomal DNA containing a mutation to sulfonamide resistance.
The gene products, SulB and SulC, correspond to polypeptides of 49 and 21
kDa, respectively. SulC has GTP cyclohydrolase activity and catalyzes the
first step in the folate biosynthetic pathway. SulB apparently has
dihydrofolate synthetase activity in that it complements a folC mutant of
Escherichia coli and thus catalyzes the last step in the pathway. Prior
work showed that SulA, a dihydropteroate synthase, and SulD, a bifunctional
enzyme, catalyze three intervening steps. Mapping of the mRNA transcribed
from the operon was consistent with its beginning at a promoter with a -35
site (gTGtCc) and an extended -10 site (T-TG-TAaAAT) and its termination at
the end of a hairpin structure, which would give a transcript 3,745
nucleotides in length. SulC showed a considerable conservation of sequence
by comparison with proven or putative GTP cyclohydrolases from four
unrelated species, with 38 to 53% of the residues being identical. A
similar comparison of SulB with dihydrofolate synthetases showed an
identity of only 26 to 37%. Overall, comparisons of the five folate
biosynthetic enzymes in different species suggest that S. pneumoniae is
related more closely to other gram-positive bacteria, less closely to
eucaryotes, and least closely to the gram-negative E. coli. The varied
arrangements of folate biosynthetic genes in different species imply an
early evolutionary period of fluidity in genomic rearrangement.
Copyright © 1995, American Society for Microbiology
A cluster of four genes encoding enzymes for five steps in the folate biosynthetic pathway of Streptococcus pneumoniae
Biology Department, Brookhaven National Laboratory, Upton, New York 11973.
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