e 393 wau??n N no d h no.cro P.☐ 99 Cm N Po d P Ppo.Ge P.a ↑ cl N P/O.cl P P/o,cro P cn 90● cH N PIO.d P/D,cro CH O P O S R A W u tio has pressor and the estab from degradation by a host enzyme,the Prescott−Harley−Klein: Microbiology, Fifth Edition VI. The Viruses 17. The Viruses: Bacteriophages © The McGraw−Hill Companies, 2002 also will be inhibited. It should be noted that immunity always in￾volves repressor activity. A potential host cell might remain unin￾fected due to a mutation that alters its phage receptor site. In such an instance it is said to be resistant, not immune, to the phage. The sequence of events leading to the initial synthesis of re￾pressor and the establishment of lysogeny is well known. Imme￾diately after lambda DNA has been circularized and transcription has commenced, the cII and cIII proteins accumulate (figure 17.18a). The cII protein binds next to the promoter for the cII gene (PRE, RE stands for repressor establishment), and stimulates RNA polymerase binding (figure 17.18b). The cIII protein protects cII from degradation by a host enzyme, the HflA protease. Lambda 17.5 Temperate Bacteriophages and Lysogeny 393 gpclll gpN clll N PL/OL cl PRM PR/OR cro PRE cll gpcll gpcro clll N PL/OL cl PRM PR/OR cro PRE cll Repressor clll N PL /OL cl PRM PR/OR cro PRE cll clll N PL/OL cl cll PR/OR QS R Late gene expression gpQ gpcro gpcro cro O AW P Figure 17.18 Choice Between Lysogeny and Lysis. Events involved in the choice between the establishment of lysogeny and continuation of the lytic cycle. The action of gpN is ignored for the sake of simplicity, and the scale in part (d) differs from that in parts (a) to (c). The abbreviation gp stands for gene product (gpcro is the product of the cro gene). (a) and (b) illustrate the initial steps leading to lambda repressor synthesis. (c) represents the situation when repressor production overcomes cro synthesis and establishes lysogeny. In part (d) the cro protein has accumulated more rapidly than lambda repressor and cro protein dimers (the active form) have bound to OL and OR. This blocks both cI and cro gene function, but not late gene expression, since gpQ has already accumulated and promoted late mRNA synthesis. See text for further details. (a) (b) (c) (d)