Harpers

REGULATION OF GENE EXPRESSION / 381A B C DCOOHNH 2Amino acids132 – 236Amino acids1 – 92COOH COOHCOOH COOHNH 2 NH 2 NH 2 NH 2CroO R 1 O R 3Figure 39–6. Schematic molecular structures of cI (lambda repressor, shown in A, B, and C)and Cro (D). Lambda repressor protein is a polypeptide chain 236 amino acids long. The chainfolds itself into a dumbbell shape with two substructures: an amino terminal (NH 2 ) domain anda carboxyl terminal (COOH) domain. The two domains are linked by a region of the chain that issusceptible to cleavage by proteases (indicated by the two arrows in A). Single repressor molecules(monomers) tend to associate to form dimers (B); a dimer can dissociate to formmonomers again. A dimer is held together mainly by contact between the carboxyl terminaldomains (hatching). Repressor dimers bind to (and can dissociate from) the recognition sites inthe operator region; they display the greatest affinity for site O R 1 (C). It is the amino terminaldomain of the repressor molecule that makes contact with the DNA (hatching). Cro (D) has asingle domain with sites that promote dimerization and other sites that promote binding ofdimers to operator, preferentially to O R 3. (Reproduced, with permission, from Ptashne M, JohnsonAD, Pabo CO: A genetic switch in a bacterial virus. Sci Am [Nov] 1982;247:128.)stabilize this state of differentiation. In the event that intracellularrepressor protein concentration becomes veryhigh, this excess repressor will bind to O R 3 and by sodoing diminish transcription of the repressor gene fromthe leftward promoter until the repressor concentrationdrops and repressor dissociates itself from O R 3.With such a stable, repressive, cI-mediated, lysogenicstate, one might wonder how the lytic cycle couldever be entered. However, this process does occur quiteefficiently. When a DNA-damaging signal, such as ultravioletlight, strikes the lysogenic host bacterium,fragments of single-stranded DNA are generated thatactivate a specific protease coded by a bacterial geneand referred to as recA (Figure 39–7). The activatedrecA protease hydrolyzes the portion of the repressorprotein that connects the amino terminal and carboxylterminal domains of that molecule (see Figure 39–6A).Such cleavage of the repressor domains causes the repressordimers to dissociate, which in turn causes dissociationof the repressor molecules from O R 2 andeventually from O R 1. The effects of removal of repressorfrom O R 1 and O R 2 are predictable. RNA polymeraseimmediately has access to the rightward promoterand commences transcribing the cro gene, and the enhancementeffect of the repressor at O R 2 on leftwardtranscription is lost (Figure 39–7).The resulting newly synthesized Cro protein alsobinds to the operator region as a dimer, but its order ofpreference is opposite to that of repressor (Figure39–7). That is, Cro binds most tightly to O R 3, butthere is no cooperative effect of Cro at O R 3 on thebinding of Cro to O R 2. At increasingly higher concentrationsof Cro, the protein will bind to O R 2 and eventuallyto O R 1.Occupancy of O R 3 by Cro immediately turns offtranscription from the leftward promoter and in thatway prevents any further expression of the repressorgene. The molecular switch is thus completely“thrown” in the lytic direction. The cro gene is now expressed,and the repressor gene is fully turned off. Thisevent is irreversible, and the expression of other lambdagenes begins as part of the lytic cycle. When Cro repressorconcentration becomes quite high, it will eventuallyoccupy O R 1 and in so doing reduce the expression of itsown gene, a process that is necessary in order to effectthe final stages of the lytic cycle.The three-dimensional structures of Cro and of thelambda repressor protein have been determined byx-ray crystallography, and models for their binding and effectingthe above-described molecular and genetic eventshave been proposed and tested. Both bind to DNA usinghelix-turn-helix DNA binding domain motifs (see below).