The mechanism of transcriptional activation by the topologically DNA-linked sliding clamp of bacteriophage T4.
Three viral proteins participate directly in transcription of bacteriophage T4 late genes: the sigma-family protein gp55 provides promoter recognition, gp33 is the co-activator, and gp45 is the activator of transcription; gp33 also represses transcription in the absence of gp45. Transcriptional activation by gp45, the toroidal sliding clamp of the T4 DNA polymerase holoenzyme, requires assembly at primer-template junctions by its clamp loader. The mechanism of transcriptional activation has been analyzed by examining rates of formation of open promoter complexes. The basal gp55-RNA polymerase holoenzyme is only weakly held in its initially formed closed promoter complex, which subsequently opens very slowly. Activation ( approximately 320-fold in this work) increases affinity in the closed complex and accelerates promoter opening. Promoter opening by gp55 is also thermo-irreversible: the T4 late promoter does not open at 0 degrees C, but once opened at 30 degrees C remains open upon shift to the lower temperature. At a hybrid promoter for sigma(70) and gp55-holoenzymes, only gp55 confers thermo-irreversibility of promoter opening. Interaction of gp45 with a C-terminal epitope of gp33 is essential for the co-activator function of gp33.
Division of Biology and Center for Molecular Genetics, University of California, San Diego, 9500 Gilman Drive, La Jolla 92093-0634, USA. skoles1@uic.edu
Scott E
SE
Kolesky
Mohamed
M
Ouhammouch
E Peter
EP
Geiduschek
D017122:Bacteriophage T4 / Q000235:genetics*; D001483:Base Sequence; D004279:DNA, Viral / Q000235:genetics / Q000378:metabolism; D015967:Gene Expression Regulation, Viral; D007700:Kinetics; D008024:Ligands; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D017434:Protein Structure, Tertiary; D012808:Sigma Factor / Q000494:pharmacology; D013696:Temperature; D015534:Trans-Activators / Q000737:chemistry / Q000494:pharmacology; D015533:Transcriptional Activation; D014764:Viral Proteins / Q000737:chemistry / Q000494:pharmacology
D004279:DNA, Viral; D008024:Ligands; D046911:Macromolecular Substances; D012808:Sigma Factor; D015534:Trans-Activators; D014764:Viral Proteins; C085859:gene 33 protein, Enterobacteria phage T4; C074555:gene 45 protein, Enterobacteria phage T4; C106892:gene 55 protein, Enterobacteria phage T4