CONSTITUTIVE ATP HYDROLYSIS AND TRANSCRIPTION ACTIVATION BY A STABLE,TRUNCATED FORM OF RHIZOBIUM-MELILOTI DCTD, A O(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 la cks 142 amino acid residues from the N terminus (designated DCTDL143). Purified DCTDL143 recognized the DCTD-binding sites at the dctA promo ter region and catalyzed the isomerization of closed complexes between sigma(54)-holoenzyme and the dctA promoter to open complexes. Like th e related sigma(54)-dependent activators NTRC and NIFA a purine nucleo side triphosphate with a hydrolyzable beta-gamma bond was required pri or to transcription initiation for this isomerization. DCTDL143 hydrol yzed purine nucleoside triphosphates but not pyrimidine nucleoside tri phosphates. As observed with NTRC-phosphate, the specific activity for the ATPase of DCTDL143 was strongly dependent on the enzyme concentra tion and was stimulated by DNA fragments bearing the binding sites for the protein. These DNA fragments increased the V-max for MgATP hydrol ysis but did not significantly lower the apparent K-m for MgATP. These data are consistent with the idea proposed for related activators tha t DCTDL143 must assemble into an active, oligomeric form before it can hydrolyze MgATP and presumably activate transcription.