The E. coli bacteriophage T4 has fifteen homing endonuclease gene insertions, five of which are predicted to be free-standing HNH homing endonucleases, named mobA to mobE. The focus of my studies is the mobE homing endonuclease gene found inserted between the conserved aerobic ribonucleotide reductase large (nrdA) and small subunits (nrdB). I showed that mobE is expressed during T4 infection, and a conserved Rho- independent transcriptional terminator regulates mobE expression. I demonstrated that mobE is a functional homing endonuclease gene that introduces a nick on the non-coding strand of the T2 nrdB gene. In T2xT4 mixed infections, mobE is inherited in -91.3% of the progeny and the inheritance was reduced to -72.8% in infections with T4 bearing a mobE internal deletion of the predicted HNH motif. Consequently, MobE activity also causes trans homing of the defunct I-TevIII homing endonuclease gene encoded within an intron interrupting the T2 nrdB gene
