ESCHERICHIA-COLI SSB ACTIVATES N4 VIRION RNA-POLYMERASE PROMOTERS BY STABILIZING A DNA HAIRPIN REQUIRED FOR PROMOTER RECOGNITION

Bacteriophage N4 virion RNA polymerase transcription of double-strande d promoter-containing DNAs requires supercoiled template and E. coli s ingle-stranded DNA-binding protein (EcoSSB); other single-stranded DNA -binding proteins cannot substitute. The DNA determinants of virion RN A polymerase binding at the promoter comprise a small template-strand hairpin. The requirement for EcoSSB is surprising, since single-strand ed DNA-binding proteins destabilize hairpin structures. DNA footprinti ng of EcoSSB on wild-type and mutant promoters indicates that EcoSSB s tabilizes the template-strand hairpin owing to the hairpin-loop sequen ces. Other single-stranded DNA-binding proteins destabilize the promot er hairpin, explaining the specificity of EcoSSB activation. We conclu de that EcoSSB activates transcription by providing the appropriate DN A structure for polymerase binding. The existence of small hairpins st able to single-stranded protein binding suggests a novel mechanism tha t provides structural determinants for specific recognition in single- stranded DNA transactions by an otherwise nonspecific DNA-binding prot ein.