Sw. Hutcheson et al., Enhancer-binding proteins HrpR and HrpS interact to regulate hrp-encoded type III protein secretion in Pseudomonas syringae strains, J BACT, 183(19), 2001, pp. 5589-5598
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 iden
tified 5 ' to hrpR, and reverse transcriptase PCR detected the presence of
an hrpRS transcript. The hrpR promoter and coding sequence were conserved a
mong P. syringae strains. The coding sequences for hrpR and hrpS were clone
d into compatible expression vectors, and their activities were monitored i
n Escherichia coli transformants carrying an hrpL ' -lacZ fusion. HrpS coul
d function as a weak activator of the hrpL promoter, but the activity was o
nly 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 bot
h HrpR and HrpS in the activation of the hrpL promoter. By using a yeast tw
o-hybrid assay, an interaction between HrpR and HrpS was detected, suggesti
ve 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 sigma (54)-dependent hrp
L promoter in P. syringae strains.