mwm M^MMftVMiHi u uu u g

FIGURE 16-37 ONA site and transcribed RNA structures active in retroregulation of int expression. At the top ts shown the DNA sequence and. below, the small cylinders shew the symmetric sequences that form hairpins in RIMA. The structure on the left shows the terminator formed in RNA transcribed from P without antitermination by M protern, which is resistant to degradation by nucleases Thc-structure on the right shows an extended loop formed in RNA transcribed from P under the influence of M protein antiterminator, which is a target for cleavage by RNAse 111 and degradation by nucleases.

There is yet a further subtlety in this regulatory device. When a prophage is induced, it needs to make integrase (together with another enzyme, called excisionase; see Chapter 11) to catalyze reformation of free phage DNA by recombination out of the bacterial DNA; and it must do this whether or not Cll activity is high. Thus, under these circumstances, the phage must make stable integrase mRNA from /\ despite the antitermination activity of N protein. How is this achieved?

When the phage genome is integrated into the bacterial chromosome during the establishment of lysogeny, the phage attachment site at which recombination occurs is between the end of the int gene and those sequences encoding the extended stem from which mRNA degradation is begun (see Figure 16-25). Thus, in the integrated form, the site causing degradation is removed from the end of the int gene, and so int mRNA made from PL is stable.

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