Regulation of int gene expression. Transcripts of the integrase gene (int) are generated from two promoters, P_I and P_L. The P_I-derived transcript is short, ending at a terminator structure that follows the coding sequence of the int gene. P_L-derived transcripts are longer and are generated as a result of N protein–mediated antitermination. The 3′ end of the P_L-derived transcript extends beyond the terminator. A structure at the 3′ end makes the transcript a substrate for cellular ribonucleases, and the transcript is degraded. Thus, more integrase protein is generated from the P_I-derived transcript than from the P_L-derived transcript.