Sensor I Threonine of the AAA+ ATPase Transcriptional Activator PspF Is Involved in Coupling Nucleotide Triphosphate Hydrolysis to the Restructuring of σ54-RNA Polymerase

Abstract

Transcriptional initiation invariably involves the transition from a closed RNA polymerase (RNAP) promoter complex to a transcriptional competent open complex. Activators of the bacterial sigma(54)-RNAP are AAA+ proteins that couple ATP hydrolysis to restructure the sigma(54)-RNAP promoter complex. Structures of the sigma(54) activator PspF AAA+ domain (PspF(1-275)) bound to sigma(54) show two loop structures proximal to sigma(54) as follows: the sigma(54) contacting the GAFTGA loop 1 structure and loop 2 that classifies sigma(54) activators as pre-sensor 1 beta-hairpin AAA+ proteins. We report activities for PspF(1-275) mutated in the AAA+ conserved sensor I threonine/asparagine motif (PspF(1-275)(T148A), PspF(1-275)(N149A), and PspF(1-275)(N149S)) within the second region of homology. We show that sensor I asparagine plays a direct role in ATP hydrolysis. However, low hydrolysis rates are sufficient for functional output in vitro. In contrast, PspF(1-275)(T148A) has severe defects at the distinct step of sigma(54) promoter restructuring. This defect is not because of the failure of PspF(1-275)(T148A) to stably engage with the closed sigma(54) promoter, indicating (i) an important role in ATP hydrolysis-associated motions during energy coupling for remodeling and (ii) distinguishing PspF(1-275)(T148A) from PspF(1-275) variants involved in signaling to the GAFTGA loop 1, which fail to stably engage with the promoter. Activities of loop 2 PspF(1-275) variants are similar to those of PspF(1-275)(T148A) suggesting a functional signaling link between Thr(148) and loop 2. In PspF(1-275) this link relies on the conserved nucleotide state-dependent interaction between the Walker B residue Glu(108) and Thr(148). We propose that hydrolysis is relayed via Thr(148) to loop 2 creating motions that provide mechanical force to the GAFTGA loop 1 that contacts sigma(54).