Expression, purification, crystallization and preliminary X‐ray diffraction analysis of a lactococcal bacteriophage small terminase subunit

Terminases are enzymes that are required for the insertion of a single viral genome into the interior of a viral procapsid by a process referred to as `encapsulation or packaging'. Many double‐stranded DNA viruses such as bacteriophages T3, T4, T7, λ and SPP1, as well as herpes viruses, utilize terminase enzymes for this purpose. All the terminase enzymes described to date require two subunits, a small subunit referred to as TerS and a large subunit referred to as TerL, for in vivo activity. The TerS and TerL subunits interact with each other to form a functional hetero‐oligomeric enzyme complex; however the stoichiometry and oligomeric state have not been determined. We have cloned, expressed and purified recombinant small terminase TerS from a 936 lactococcal bacteriophage strain ASCC454, initially isolated from a dairy factory. The terminase was crystallized using a combination of nanolitre sitting drops and vapour diffusion using sodium malonate as the precipitant, and crystallization optimized using standard vapour‐diffusion hanging drops set up in the presence of a nitrogen atmosphere. The crystals belong to the P 2 space group, with unit‐cell parameters a = 73.93, b = 158.48, c = 74.23 Å, and diffract to 2.42 Å resolution using synchrotron radiation. A self‐rotation function calculation revealed that the terminase oligomerizes into an octamer in the asymmetric unit, although size‐exclusion chromatography suggests that it is possible for it to form an oligomer of up to 13 subunits.