_____________________________________________________________________ Figure 1. Interaction of the T4 portal with DNA. _____________________________________________________________________ Fitting of a B-DNA structure into the central channel of the dodecameric portal assembly shows that the contacts between any one gp20 molecule and the DNA are confined to three polypeptide loops separated from each other by approximately one DNA helical turn. These occur at the end of the clip domain (inner clip loop), at the loop near the amino end of a7 in the stem domain (annotation No.5) and at the tunnel loop. _____________________________________________________________________ The inner clip loop contains two basic residues (R338 and K342) that are present in each of the 12 portal protein monomers, pointing into the DNA channel (annotation No. 3). These 24 residues surround the entrance of the annotation No. 1, suggesting that these positive charges might be important to capture the start of the viral genome to initiate DNA packaging.