Histone-like protein HU as a specific transcriptional regulator: co-factor role in repression of gal transcription by GAL repressor.
BACKGROUND
Transcription initiation from the two overlapping promoters of the gal operon in Escherichia coli is negatively regulated by binding of Gal repressor (GalR) to bipartite operators, which encompass the promoters. Coordinated repression of the two promoters requires GalR binding to both operators. In a purified system, GalR, nevertheless, fails to show the coordinated repression, predicting the participation of an additional factor(s) in the regulation in vivo.


RESULTS
We have purified a protein that restored the expected GalR-mediated repression for the in vitro system and have identified this factor to be the bacterial histone-like protein HU. In vitro transcription assays in the presence of GalR and HU show that, just as in vivo, the coordinated repression of the two gal promoters requires GalR binding to both operators and is sensitive to the inducer, D-galactose. The GalR and HU dependent repression also requires supercoiled DNA template and prevents open complex formation.


CONCLUSIONS
We propose that HU, acting as a co-factor, brings about the GalR-mediated repression by forming a distinct nucleoprotein complex of higher order structure. Although how HU participates in the assembly process is unknown, there may be a cooperative effect in the formation of the repression complex.
Aki
T
T
Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892-4255, USA.
Choy
H E
HE
Adhya
S
S
D001426:Bacterial Proteins / Q000378:metabolism*; D018983:DNA Footprinting; D004278:DNA, Superhelical / Q000235:genetics / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics*; D029968:Escherichia coli Proteins; D005690:Galactose / Q000096:biosynthesis*; D015964:Gene Expression Regulation, Bacterial; D008957:Models, Genetic; D009154:Mutation; D009690:Nucleic Acid Conformation; D009876:Operon / Q000235:genetics*; D012097:Repressor Proteins / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism*; D014158:Transcription, Genetic
D001426:Bacterial Proteins; D004278:DNA, Superhelical; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; C050993:Galactose repressor proteins; D012097:Repressor Proteins; D014157:Transcription Factors; C040330:histone-like protein HU, bacteria; D005690:Galactose