Morphogenesis of bacteriophage P22 involves the packaging of double-stranded DNA into a preassembled procapsid. DNA is translocated by a powerful virally encoded molecular motor called terminase, which comprises large (gp2, 499 residues) and small (gp3, 162 residues) subunits. While gp2 contains the phosphohydrolase and endonuclease activities of terminase, the function of gp3 may be to regulate specific and nonspecific modes of DNA recognition as well as the enzymatic activities of gp2. Electron microscopy shows that wild-type gp3 self-assembles into a stable and monodisperse nonameric ring. A three-dimensional reconstruction at 18 A resolution provides the first glimpse of P22 terminase architecture and implies two distinct modes of interaction with DNA-involving a central channel of 20 A diameter and radial spikes separated by 34 A. Electromobility shift assays indicate that the gp3 ring binds double-stranded DNA nonspecifically in vitro via electrostatic interactions between the positively charged C-terminus of gp3 (residues 143-152) and phosphates of the DNA backbone. Raman spectra show that nonameric rings formed by subunits truncated at residue 142 retain the subunit fold despite the loss of DNA-binding activity. Difference density maps between gp3 rings containing full-length and C-terminally truncated subunits are consistent with localization of residues 143-152 along the central channel of the nonameric ring. The results suggest a plausible molecular mechanism for gp3 function in DNA recognition and translocation.