Bacteriophage λ DNA packaging: DNA site requirements for termination and processivity

Bacteriophage λ chromosomes are processively packaged into preformed shells, using end-to-end multimers of intracellular viral DNA as the packaging substate. A 200 bp long DNA segment, cos, contains all the sequences needed for DNA packaging. The work reported here shows that efficient DNA packaging termination requires cos‘s I2 segment, in addition to the required termination subsite, cosQ, and the nicking site, cosN. Efficient processivity requires cosB, in addition to cosQand cosN. An initiation-defective mutant form of cosBsponsored efficient processivity, indicating that the terminase-cosB interactions required for termination are less stringent than those required at initiation. The finding that an initiation-defective form of cosB is functional for processivity allows a re-interpretation of a similar finding, obtained previously, that the initiation-defective cosB of phage 21 is functional for processivity by the λ packaging machinery. The cosBφ21 result can now be interpreted as indicating that interactions between cosBφ21 and λ terminase, while insufficient for initiation, function for processivity.