Journal of Bacteriology, October 1998, p. 5151-5158, Vol. 180, No. 19
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
andDepartment of Molecular and Cell Biology, University of California, Berkeley, California 94720-3202
Received 11 June 1998/Accepted 3 August 1998
The sid gene promoter (Psid),
which controls expression of the late genes from satellite phage P4, is
activated by a unique class of small DNA-binding proteins. The
activators from both satellite and helper phages stimulate
transcription from Psid. These activators bind
to sites centered at position
55 in all the helper and satellite
phage late promoters. P4 Psid is unique in that
it has an additional activator binding site centered at position
18
(site II). We have constructed a mutant of site II that no longer binds
activators. Transcription under the control of satellite phage
activators is increased by the site II mutation. In contrast, helper
phage activators do not show this increase in transcription from
Psid mutated at site II. Competition gel shift
analysis reveals that the P4 satellite phage activator, Delta, binds
eightfold better to site II than to site I. The products of the
sid transcription unit are needed only when a helper phage
is present; thus, the satellite phage activators repress transcription
until the helper is present to supply a nonrepressing activator.
Present address: Kosan Biosciences, Burlingame, CA 94010.
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