The C-terminal domain of the bacteriophage T4 terminase docks on the prohead portal clip region during DNA packaging.

Bacteriophage ATP-based packaging motors translocate DNA into a pre-formed prohead through a dodecameric portal ring channel to high density. We investigated portal-terminase docking interactions at specifically localized residues within a terminase-interaction region (aa279-316) in the phage T4 portal protein gp20 equated to the clip domain of the SPP1 portal crystal structure by 3D modeling. Within this region, three residues allowed A to C mutations whereas three others did not, consistent with informatics analyses showing the tolerated residues are not strongly conserved evolutionarily. About 7.5nm was calculated by FCS-FRET studies employing maleimide Alexa488 dye labeled A316C proheads and gp17 CT-ReAsH supporting previous work docking the C-terminal end of the T4 terminase (gp17) closer to the N-terminal GFP-labeled portal (gp20) than the N-terminal end of the terminase. Such a terminase-portal orientation fits better to a proposed "DNA crunching" compression packaging motor and to portal determined DNA headful cutting.