Infection of Escherichia coli by the T7 bacteriophage leads to the rapid and
selective inhibition of the host RNA polymerase (RNAP) - a multisubunit enzyme
responsible for gene transcription - by a small (7 kDa) phage-encoded
protein called Gp2 (Gene 2 protein). Gp2 is also a potent inhibitor of E. coli
RNAP in vitro. Here, we provide the first structural insight into Gp2. The
structure of Gp2 revealed a distinct separation of positive and negative surface
charges on different sides of the molecule. The two highly exposed arginines
are also in agreement with the mutagenesis studies, which suggested that they
have an important role on the Gp2-RNAP interaction and Gp2's function. We
have also provided the structural insight into a small domain of the RNAP β'
subunit, the β' jaw domain, which is known from previous mutagenesis studies
to possess the site for Gp2 binding. Evidence for an interaction between Gp2
and β' jaw domain were provided by Nuclear Magnetic Resonances (NMR)
titration experiments. The Gp2-Jaw complex structure solved by NMR spectroscopy
has largely facilitated the elucidation of Gp2's inhibition mechanism
by allowing the construction of a Gp2-RNAP complex model, which also suggested
other possible interaction sites between RNAP and Gp2. Furthermore,
the new selective methyl labelling methods may provide experimental data for
interactions between Gp2 and RNAP by NMR.
RNAP has been used as an important target for broad-spectrum antibacterial
therapy and for anti-tuberculosis therapy. According to the structural
model of the Gp2-RNAP complex, the binding site of Gp2 is distal to the
catalytic cleft, which does not overlap with the binding site of the antibiotic
Rifamycin. Understanding the inhibition mechanism of Gp2 may thus open
up a new route for the development of RNAP targeted antibiotics.