ABSTRACT
The Escherichia coli rhaSR operon encodes two AraC family transcription activators, RhaS and RhaR, and is activated by RhaR in the presence of l-rhamnose. β-Galactosidase assays of various rhaS-lacZ promoter fusions combined with mobility shift assays indicated that a cyclic AMP receptor protein (CRP) site located at −111.5 is also required for full activation of rhaSR expression. To address the mechanisms of activation by CRP and the RNA polymerase α-subunit C-terminal domain (α-CTD) at rhaSR, we tested the effects of alanine substitutions in CRP activating regions 1 and 2, overexpression of a truncated version of α (α-Δ235), and alanine substitutions throughout α-CTD. We found that DNA-contacting residues in α-CTD are required for full activation, and for simplicity, we discuss α-CTD as a third activator of rhaSR. CRP and RhaR could each partially activate transcription in the absence of the other two activators, and α-CTD was not capable of activation alone. In the case of CRP, this suggests that this activation involves neither an α-CTD interaction nor cooperative binding with RhaR, while in the case of RhaR, this suggests the likelihood of direct interactions with core RNA polymerase. We also found that CRP, RhaR, and α-CTD each have synergistic effects on activation by the others, suggesting direct or indirect interactions among all three. We have some evidence that the α-CTD–CRP and α-CTD–RhaR interactions might be direct. The magnitude of the synergistic effects was usually greater with just two activators than with all three, suggesting possible redundancies in the mechanisms of activation by CRP, α-CTD, and RhaR.
Requirement for two copies of RNA polymerase alpha subunit C-terminal domain for synergistic transcription activation at complex bacterial promoters