FIG. 2.

Regulation of int expression by RNase III cleavage of sib. Two segments of the λ chromosome with relevant genes and sites are indicated by the two horizontal lines. Transcripts that initiate at P_L are antiterminated following binding of N protein to the nutL site (see the text). These transcripts are cleaved at sib by RNase III (indicated by the scissors symbol), and upstream RNA, which includes the Int-coding region, is destabilized by exonucleolytic digestion in a 3′-to-5′ direction (indicated by the “Pacman” symbol). If sufficient CII protein is made, it activates transcription at P_Int, a promoter whose start site is within xis. These transcripts terminate at the T_Int terminator (indicated by the hairpin symbol), do not contain a complete sib, are not cleaved by RNase III, and are relatively resistant to exonucleolytic digestion.