Journal of Bacteriology, October 2001, p. 5589-5598, Vol. 183, No. 19
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.19.5589-5598.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

andDepartment of Cell Biology and Molecular Genetics, University of Maryland, College Park, Maryland 20742
Received 9 May 2001/Accepted 6 July 2001
In Pseudomonas syringae strains, the
hrp-hrc pathogenicity island
consists of an HrpL-dependent regulon that encodes a type III protein
translocation complex and translocated effector proteins required for
pathogenesis. HrpR and HrpS function as positive regulatory factors for
the hrpL promoter, but their mechanism of action has not
been established. Both HrpR and HrpS are structurally related to
enhancer-binding proteins, but they lack receiver domains and do not
appear to require a cognate protein kinase for activity. hrpR and hrpS were shown to be expressed
as an operon: a promoter was identified 5' to hrpR, and
reverse transcriptase PCR detected the presence of an
hrpRS transcript. The hrpR promoter and
coding sequence were conserved among P. syringae
strains. The coding sequences for hrpR and
hrpS were cloned into compatible expression vectors, and
their activities were monitored in Escherichia coli transformants carrying an hrpL'-lacZ
fusion. HrpS could function as a weak activator of the
hrpL promoter, but the activity was only 2.5% of the
activity detected when both HrpR and HrpS were expressed in the
reporter strain. This finding is consistent with a requirement for both
HrpR and HrpS in the activation of the hrpL promoter. By
using a yeast two-hybrid assay, an interaction between HrpR and
HrpS was detected, suggestive of the formation of a heteromeric
complex. Physical interaction of HrpR and HrpS was confirmed by
column-binding experiments. The results show that HrpR and HrpS
physically interact to regulate the
54-dependent
hrpL promoter in P. syringae strains.
Present address: Department of Physiology, The Johns Hopkins
University School of Medicine, Baltimore, MD 21205-2105.
Present address: Department of Medical Genetics and Microbiology,
Toronto, Ontario, Canada M5S 1A8.
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