The maturation of coliphage lambda DNA in the absence of its packaging

In vivo, λ DNA cannot be cleaved at cos (matured) if proheads are not present; in vitro, however, cos cleavage readily takes place in the absence of proheads. In order to investigate this paradox, we have constructed plasmids that synthesize λ terminase in vivo upon induction. The plasmids also contain cos at the normal position, about 190 bp upstream of λ gene Nul. One of the plasmids, pFM3, produces levels of terminase comparable to those found after phage induction. If cells carrying pFM3 are thermoinduced, almost 100% of the intracellular plasmid DNA has a double-strand interruption at or near cos.

Since the only λ genes that pFM3 carries are Nul, A, W and B, this in vivo cleavage is occurring in the absence of proheads. Previous failure to observe 2 maturation with phages carrying prohead mutations may be due to exonucleolytic degradation of the unprotected DNA ends, a different DNA topology or compartmentalization, or terminase inhibition in the absence of prohead by the product of another λ gene that maps to the right of gene B.