The Nu1 subunit of bacteriophage lambda terminase.

The maturation and packaging of bacteriophage lambda DNA are catalyzed by the phage terminase enzyme. Terminase is composed of two protein subunits, gpNu1 and gpA. The holoenzyme is multifunctional in vitro; it binds to and cleaves lambda DNA at the cos site (where cos represents cohesive-end site), packages DNA into lambda proheads, and is also a DNA-dependent ATPase. The genes of the two subunits have been cloned separately into powerful expression vectors which allow for very high levels of protein overproduction. The gpNu1 protein has been purified to homogeneity and has a monomeric molecular weight of 21,200, in close agreement with the Mr of 20,444 expected from its amino acid sequence. Both gel filtration and sedimentation velocity centrifugation indicate that the native gpNu1 protein exists as a Mr greater than 500,000 aggregate. The sequence of the first 20 amino acids and the overall composition both match those predicted by the nucleotide sequence of the Nu1 gene. Purified gpNu1 is able to complement gpA-containing extracts in both lambda DNA packaging and cos cleavage assays. The Nu1 gene amino acid sequence predicts DNA binding by the protein, and gpNu1 does show specific binding to lambda DNA by filter binding assays. Also, as predicted from its sequence, gpNu1 exhibits ATPase activity; but in contrast to the holoenzyme, this activity is DNA-independent.