The Basal Transcription Factors TBP and TFB from the Mesophilic Archaeon Methanosarcina mazeii: Structure and Conformational Changes upon Interaction with Stress-gene Promoters

Transcription of archaeal non-stress genes involves the basal factors TBP and TFB, homologs of the eucaryal TATA-binding protein and transcription factor IIB, respectively. No comparable information exists for the archaeal molecular-chaperone, stress genes hsp70(dnaK), hsp40(dnaJ), and grpE. These do not occur in some archaeal species, but are present in others possibly due to lateral transfer from bacteria, which provides a unique opportunity to study regulation of stress-inducible bacterial genes in organisms with eukaryotic-like transcription machinery. Among the Archaea with the genes, those from the mesophilic methanogen Methanosarcina mazeii are the only ones whose basal (constitutive) and stress-induced transcription patterns have been determined. To continue this work, tbp and tfb were cloned from M. mazeii , sequenced, and the encoded recombinant proteins characterized in solution, separately and in complex with each other and with DNA. M. mazeii TBP ranks among the shortest within Archaea and, contrary to other archaeal TBPs, it lacks tryptophan or an acidic tail at the C terminus and has a basic N-terminal third. M. mazeii TFB is similar in length to archaeal and eucaryal homologs and all have a zinc finger and HTH motifs. Phylogenetically, the archaeal and eucaryal proteins form separate clusters and the M. mazeii molecules are closer to the homologs from Archaeoglobus fulgidus than to any other. Antigenically, M. mazeii TBP and TFB are close to archaeal homologs within each factor family, but the two families are unrelated. The purified recombinant factors were functionally active in a cell-free in vitro transcription system, and were interchangeable with the homologs from Methanococcus thermolithotrophicus. The M. mazeii factors have a similar secondary structure by circular dichroism (CD). The CD spectra changed upon binding to the promoters of the stress genes grpE, dnaK, and dnaJ, with the changes being distinctive for each promoter; in contrast, no effect was produced by the promoter of a non-stress-gene. Factor(s)-DNA modeling predicted that modifications of H bonds are caused by TBP binding, and that these modifications are distinctive for each promoter. It also showed which amino acid residues would contact an extended TATA box with a B recognition element, and evolutionary conservation of the TBP-TFB-DNA complex orientation between two archaeal organisms with widely different optimal temperature for growth (37 and 100 °C).