Nucleic Acid Packaging of DNA Viruses

Abstract

Bacteriophages are viruses whose hosts are bacteria. Bacteriophages are the most numerous and genomically diverse form of life in the environment. Many bacteriophage genes have either (1) no detectable homology to any other bacteriophage/cellular gene or (2) homology to extremely diverged bacteriophage genes of presumably similar function. When infecting a host bacterial cell, a double-stranded DNA bacteriophage produces proteins that package progeny DNA genomes in a protein container (capsid) that protects its genome when the bacteriophage is outside of a cell. The energy requirement for DNA packaging is approximately 0.5 kcal mol−1 of packaged nucleotide pair. The capsid and accessory proteins join to form an ATP-fueled motor that drives the DNA molecule into a cavity in the capsid. The motor packages the DNA molecule via end-first motion through a portal ring also called the connector. An accessory protein/ATPase is at least the spark plug of the motor. Bacteriophage DNA packaging motors are investigated as a prototype for understanding other biological motors. Testing for feedback control of the motor is an important current objective. Investigations of both bacteriophage DNA packaging and bacteriophage capsid assembly have produced in vitro systems that are complex. One strategy for analysis of such complex systems is observation at the level of single events (single-particle analysis). Single-particle analysis has already revealed surprising cooperativity among DNA packaging events and has also revealed a comparatively strong motor capable of generating about 40 to 70 pN of force while intermittently hesitating. Bacteriophage DNA packaging systems are useful for further developing procedures for single-particle analysis of complex biochemical systems.