Constitutive ATP hydrolysis and transcription activation by a stable, truncated form of Rhizobium meliloti DCTD, a sigma 54-dependent transcriptional activator.

The dctD gene product (DCTD) activates transcription from dctA by the sigma 54-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 DCTDL143). Purified DCTDL143 recognized the DCTD-binding sites at the dctA promoter region and catalyzed the isomerization of closed complexes between sigma 54-holoenzyme and the dctA promoter to open complexes. Like the related sigma 54-dependent activators NTRC and NIFA, a purine nucleoside triphosphate with a hydrolyzable beta-gamma bond was required prior to transcription initiation for this isomerization. DCTDL143 hydrolyzed purine nucleoside triphosphates but not pyrimidine nucleoside triphosphates. As observed with NTRC-phosphate, the specific activity for the ATPase of DCTDL143 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 Vmax for MgATP hydrolysis but did not significantly lower the apparent Km for MgATP. These data are consistent with the idea proposed for related activators that DCTDL143 must assemble into an active, oligomeric form before it can hydrolyze MgATP and presumably activate transcription.