The mechanism of DNA packaging by dsDNA viruses is not well understood in any
system. In bacteriophage P22 only five genes are required for successful
condensation of DNA within the capsid. The products of three of these genes, the
portal, scaffolding, and coat proteins, are structural components of the
precursor particle, and two, the products of genes 2 and 3, are not. The
scaffolding protein is lost from the structure during packaging, and only the
portal and coat proteins are present in the mature virus particle. These five
genes map in a contiguous cluster at the left end of the P22 genetic map. Three
additional genes, 4, 10, and 26, are required for stabilizing of the condensed
DNA within the capsid. In this report we present the nucleotide sequence of 7461
bp of P22 DNA that contains the five genes required for DNA condensation, as
well as a nonessential open reading frame (ORF109), gene 4, and a portion of
gene 10. N-terminal amino acid sequencing of the encoded proteins accurately
located the translation starts of six genes in the sequence. Despite the fact
that most of these proteins have striking analogs in the other dsDNA
bacteriophage groups, which perform highly analogous functions, no amino acid
sequence similarity between these analogous proteins has been found, indicating
either that they diverged a very long time ago or that they are the products of
spectacular convergent evolution.
