Abstract
During DNA replication by the λ-like bacteriophages, immature concatemeric DNA is produced by rolling circle replication. The concatemers are processed into mature chromosomes with cohesive ends, and packaged into prohead shells, during virion assembly. Cohesive ends are generated by the viral enzyme terminase, which introduces staggered nicks at cos, an approx. 200 bp-long sequence containing subsites cosQ, cosN and cosB. Interactions of cos subsites of immature concatemeric DNA with terminase orchestrate DNA processing and packaging. To initiate DNA packaging, terminase interacts with cosB and nicks cosN. The cohesive ends of N15 DNA differ from those of λ at 2/12 positions. Genetic experiments show that phages with chromosomes containing mismatched cohesive ends are functional. In at least some infections, the cohesive end mismatch persists through cyclization and replication, so that progeny phages of both allelic types are produced in the infected cell. N15 possesses an asymmetric packaging specificity: N15 DNA is not packaged by phages λ or 21, but surprisingly, N15-specific terminase packages λ DNA. Implications for genetic interactions among λ-like bacteriophages are discussed.
Highlights
Large DNA viruses, such as tailed bacteriophages and herpes viruses, use an ATP-powered motor to translocate viral DNA into the preformed empty shell, called the prohead or procapsid [1,2,3]
Genetics of a cohesive end mismatch: Phages with mismatched cohesive ends are viable; the mismatches can persist to post-DNA replication
Two prophages integrated in tandem at attB contain between them a chromosome that mimics a chromosome in a concatemer
Summary
Large DNA viruses, such as tailed bacteriophages and herpes viruses, use an ATP-powered motor to translocate viral DNA into the preformed empty shell, called the prohead or procapsid [1,2,3]. The prohead shell is an icosahedral lattice principally constructed of many copies of the major capsid protein whose fold is conserved [5]. Conserved in the herpes viruses and tailed bacteriophages, is usually a hetero-oligomer of small (TerS) and large (TerL) subunits [2,10,11,12]. TerL is a motor protein whose N-terminal ATPase domain powers translocation of the DNA into the prohead. TerL contains the C-terminal endonuclease domain that cuts concatemeric DNA into unit-length virion chromosomes
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