Practice question

First gene of lac polycistron, lacZ, encodes beta-galactosidase, a large tetrameric enzyme comprising 116 kDa protomers with active sites containing acidic residues Glu461 and Glu537 acting as acid-base and nucleophile in retaining glycosidase mechanism with double displacement. Catalytically it cleaves terminal beta-D-galactose from lactose and chromogenic analogues ONPG and X-gal, and performs transgalactosylation to produce allolactose inducer that regulates operon. Permease encoded by lacY transports lactose across membrane, transacetylase from lacA detoxifies non-metabolizable sugars by acetylation, and repressor from lacI regulates transcription initiation, distinct functional categories in pathway.