The P4 protein of bacteriophage 12 is a hexameric molecular motor closely related to super family 4 (SF4) helicases. P4 converts chemical energy from ATP hydrolysis into mechanical work, to translocate single stranded RNA into a viral capsid. The molecular basis of mechano-chemical coupling, i.e. how small ~1Å changes in the ATP binding site are amplified into nanometer scale motion along the nucleic acid, is not understood at atomic level. Here we study in atomic detail the mechano-chemical coupling using structural and biochemical analyses of P4 mutants. We show that a conserved region, comprising SF4 helicase motifs H3 and H4 and loop L2, constitutes the moving lever of the motor. The lever tip encompasses an RNA binding site which moves along the mechanical reaction coordinate. The lever is flanked by -phosphate sensors (Asn234 and Ser252) which report the nucleotide state of neighboring subunits and control the lever position. Insertion of an arginine finger (Arg279) into the neighboring catalytic site is concomitant with lever movement and commences ATP hydrolysis. This assures cooperative sequential hydrolysis which is tightly coupled to mechanical motion. Given the structural conservation the mutated residues may play similar roles in other hexameric helicases and related molecular motors.