Practice question

Strong promoters possess near-perfect -35 TTGACA and -10 TATAAT hexamers separated by optimal 17 base pair spacer including AT-rich UP element interacting with alpha C-terminal domain, producing exceptionally high intrinsic affinity for sigma70 holoenzyme and spontaneous open complex formation. Uncontrolled activity would wastefully consume nucleotides, ribosomes, and energy producing unnecessary transcripts. Evolution pairs potent cores with repressor proteins whose operators overlap start site, creating steric block until specific signal inactivates repressor. Net effect is high regulatory range: tightly repressed off state yet capable of extremely high output when fully derepressed for metabolic need.