Structural 3D visualization of bacteriophage T4 large terminase. This lateral surface view was visualized with UCSF ChimeraX software. The five colours represent different subunits. Large terminase is an enzyme which, together with the portal protein gp20, forms the packaging machine of the virus. The large terminase uses ATP (adenosine triphosphate) to propel two base pairs of DNA (deoxyribonucleic acid) through the portal into the capsid. Once full of DNA, the capsid is able to produce new viruses. Bacteriophages, or phages, are viruses that infect bacteria. T4 phage infects Escherichia coli bacteria.