Dual function of I-TevI. A. I-TevI binds with equal affinity to the homing site (top) and operator site (bottom). The CS sequence at the natural distance in the homing site allows endonuclease cleavage, to initiate homing. In the operator site, there is no cleavage sequence at a suitable distance, resulting in autorepression, because I-TevI binding blocks the late promoter and transcription. B. Autorepression by T4 intron-encoded endonucleases. For each of the three endonucleases, I-TevI, I-TevII, and I-TevIII, the endonuclease's homing site (HS) is aligned with proven or putative the operator site (OS) upstream of the endonuclease ORF within the td, nrdB, and nrdD introns, respectively. The operator sites are indicated by dashed boxes, with bold-type nucleotides representing identity between the operator and homing sites. The position of the endonuclease's cleavage sites are indicated by open and black triangles. Green and blue boxes indicate late and middle T4 promoters, respectively, with corresponding transcription start sites indicated by right-facing arrows labeled with the same color.