Challenging packaging limits and infectivity of phage {\lambda}

TL;DR

This study investigates the maximum genome length that bacteriophage {\lambda} can package into its capsid while remaining infectious, revealing limits influenced by DNA length and ionic conditions, with implications for viral assembly.

Contribution

The paper demonstrates that phage {\lambda} can package up to 110-114% of its wild-type DNA length, and shows how ionic conditions modulate production rates, providing new insights into packaging limits.

Findings

Phage {\lambda} can package up to 114% of WT DNA length.

Production rate decreases exponentially with increased DNA length.

Multivalent ions influence phage production efficiency.

Abstract

The terminase motors of bacteriophages have been shown to be among the strongest active machines in the biomolecular world, being able to package several tens of kilobase pairs of viral genome into a capsid within minutes. Yet these motors are hindered at the end of the packaging process by the progressive build-up of a force resisting packaging associated with already packaged DNA. In this experimental work, we raise the issue of what sets the upper limit on the length of the genome that can be packaged by the terminase motor of phage {\lambda} and still yield infectious virions, and the conditions under which this can be efficiently performed. Using a packaging strategy developed in our laboratory of building phage {\lambda} from scratch, together with plaque assay monitoring, we have been able to show that the terminase motor of phage {\lambda} is able to produce infectious particles with up to 110% of the wild-type (WT) {\lambda}-DNA length. However, the phage production rate, and thus the infectivity, decreased exponentially with increasing DNA length, and was a factor of 103 lower for the 110% {\lambda}-DNA phage. Interestingly, our in vitro strategy was still efficient in fully packaging phages with DNA lengths as high as 114% of the WT length, but these viruses were unable to infect bacterial cells efficiently. Further, we demonstrated that the phage production rate is modulated by the presence of multivalent ionic species. The biological consequences of these finding are discussed.