Practice question

Sigma70 polypeptide organized into four conserved regions comprising helical domains. Region 2, particularly subregion 2.3-2.4, responsible for recognition and melting of -10 Pribnow box element. Crystal structures show aromatic residues tryptophan from region 2 intercalate DNA, flipping adenine at -11 and thymine at -7 into protein pockets stabilizing unwound transcription bubble. Region 4.2 recognizes -35, region 1 autoinhibits DNA binding. Mutations within region 2 abolish -10 specific binding while leaving -35 interaction intact, demonstrating modular mapping. Hence region 2 primarily confers -10 element specificity essential for open complex nucleation.