Practice question

Negative regulation of trp operon occurs through dual overlapping mechanisms where repressor binding directly reduces promoter activity. Active holorepressor TrpR-tryptophan complex occupies operator site overlapping transcription start site, forming high-affinity nucleoprotein complex that sterically impedes RNA polymerase holoenzyme binding and interferes with isomerization from closed to open complex. This specific interference reduces initiation frequency approximately seventyfold. Second attenuation layer further lowers productive elongation tenfold via premature termination at leader sequence. Both regulatory layers act to down-regulate biosynthetic enzyme production when end product abundant, contrasting with positive CAP-cAMP activation required for lac catabolic activation, reflecting purely repressive control of anabolic pathways.