MBII - L18 - Transcription 2 - Transcription Regulation in Bacteria

Molecular Biology II Transcription Regulation in Bacteria Lecture 18[Page 1] The lac Operon: The first regulatory mechanism found The lac repressor mechanism controls the production of 3 proteins involved wit h lactose metabolism The lac operon contains 3 genes la cZ :-Gala ctosida se ± intr a cell ula r enzyme tha t cl ea ves la ctose p glucose + galactose la cY : L a ctose Permea se ± membr a ne-bound tr a nsport protein tha t pumps la ctose into cell la cA : Tr a ns a cetyla se ± tr a nsfers a n a cetyl group from a cetyl -CoA to -Gala ctosides The la cI gene codes for the la c repressor ± a homo-dimer The la c repressor binds tight l y to the oper a tor region within the la c operon, which is found upstre a m of the 3 genes invol ved in la ctose meta bol ism The repressor binds DNA a s a homodimer, a t 2 l oc a tions (once on ea ch str a nd) There is a palindromic sequence centred at +11 Each homodimer binds either side of this on a different strand The key repressor binding sequence (O 1 ) overlaps with the transcription start site o So repressor binding blocks transcription ± RNA P cant bind to promoter o RNAP and repressor cannot bind at the same time If lactose binds to the repressor, it can no longer bind to the op erator, which allows RNA P to transcribe the gene The repressor will bind to the main operator region O 1 (+11) and to o ne of two ancil lary operators

MBII - L18 - Transcription 2 - Transcription Regulation in Bacteria

Documents