Volume 270,
Number 21,
Issue of May 26, pp. 12339-12342, 1995
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
Divergent
Transcription and a Remote Operator Play a Role in Control of
Expression of a Nopaline Catabolism Promoter in Agrobacterium
tumefaciens
Ferenc
Marincs
,
Derek W. R.
White
The nocP-nocR divergent gene arrangement of
the nopaline catabolism (noc) operon of the Agrobacterium
tumefaciens Ti plasmid pTiT37 was examined with respect to the
expression of the nocP promoter. Under repressive conditions,
i.e. in the absence of nopaline, four distinct levels of
P
expression were observed. The lowest
level of expression, i.e. full repression, was detected in the
presence of the NocR repressor, together with the remote noc operator and productive transcription from the divergent nocR promoter. The next level was observed in the absence of either the
NocR protein or of the operator or of both. The third level was
detected when abortive transcription from the nocR promoter
occurred, irrespective of the presence or absence of the NocR protein.
The highest level of P expression was
observed in the absence of both productive transcription from
P
and the operator sequence, whether
or not the NocR protein was present. Under inductive conditions,
i.e. in the presence of nopaline, expression of
P was activated if both the NocR
protein and the operator were present. Absence of either NocR or the
operator resulted in lack of inducibility of the nocP promoter. Transcription from the divergent nocR promoter
had no influence on the activation of P. It
was also found that the absence of the operator affected plasmid
supercoiling in vivo. The results suggest that DNA topology
has a role in the regulation of the nocP promoter.
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