Constitutive ATP hydrolysis and transcription activation by a stable, truncated form of Rhizobium meliloti DCTD, a σ⁵⁴-dependent transcriptional activator

The dctD gene product (DCTD) activates transcription from dctA by the σ⁵⁴-holoenzyme form of RNA polymerase in Rhizobium meliloti. We have purified a constitutively active form of R. meliloti DCTD that lacks 142 amino acid residues from the N terminus (designated DCTD(L143)). Purified DCTD(L143) recognized the DCTD-binding sites at the dctA promoter region and catalyzed the isomerization of closed complexes between σ⁵⁴-holoenzyme and the dctA promoter to open complexes. Like the related σ⁵⁴-dependent activators NTRC and NIFA, a purine nucleoside triphosphate with a hydrolyzable β-γ bond was required prior to transcription initiation for this isomerization. DCTD(L143) hydrolyzed purine nucleoside triphosphates but not pyrimidine nucleoside triphosphates. As observed with NTRC-phosphate, the specific activity for the ATPase of DCTD(L143) was strongly dependent on the enzyme concentration and was stimulated by DNA fragments bearing the binding sites for the protein. These DNA fragments increased the V(max) for MgATP hydrolysis but did not significantly lower the apparent K(m) for MgATP. These data are consistent with the idea proposed for related activators that DCTD(L143) must assemble into an active, oligomeric form before it can hydrolyze MgATP and presumably activate transcription.