Journal of Bacteriology, August 1998, p. 3907-3916, Vol. 180, No. 15
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Department of Microbiology, Technical University of Denmark, DK2800 Lyngby, Denmark
Received 23 March 1998/Accepted 28 May 1998
A purR::pGh9:ISS1 mutant of
Lactococcus lactis was obtained following transposon
mutagenesis of strain MG1363 and selection for purine auxotrophs. After
determination of the nucleotide sequence and deduction of the
purR reading frame, the PurR product was found to be highly
similar to the purR-encoded repressor from Bacillus
subtilis. The wild-type purR gene complemented the
purine auxotrophy of a purR::ISS1
mutant, and it was shown that the
purR::ISS1 mutation lowered the level
of transcription from the purine-regulated L. lactis purD
promoter. In a parallel study on the regulation of purC and
purD expression in L. lactis (M. Kilstrup,
S. G. Jessing, S. B. Wichmand-Jørgensen, M. Madsen, and D. Nilsson, J. Bacteriol. 180:3900-3906, 1998), we identified regions
(PurBox sequences: AWWWCCGAACWWT) upstream of the promoters with a
central G residue at exactly position
76 relative to the
transcriptional start site. The PurBox sequences were found to be
required for high-level promoter activity and purine regulation. We
identified a PurBox sequence overlapping the
35 region of the
L. lactis purR promoter and found, by studies of a
purR-lacLM fusion plasmid, that purR is
autoregulated. Because of the high degree of similarity of the PurR
proteins from B. subtilis and L. lactis, we
looked for PurBox sequences in the promoter regions of the
PurR-regulated genes in B. subtilis and identified a
perfectly matching PurBox sequence in the purA promoter
region and slightly degenerate PurBox-like sequences in the promoter
regions for the pur operon and the purR gene.
Interestingly, the PurBox in the pur operon of B. subtilis is located almost identically, with respect to the
promoter, to the PurBox sequences located in front of purC
and purD in L. lactis. We present a hypothesis
to explain how an ancestral PurR protein in B. subtilis
could have evolved from an activator of the pur operon into
a repressor which regulates transcription initiation from the same
pur promoter by using the same PurR binding site and a
similar response toward its effectors.
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