MOLECULAR EVOLUTION OF BACTERIOPHAGES - DISCRETE PATTERNS OF CODON USAGE IN T4 GENES ARE RELATED TO THE TIME OF GENE-EXPRESSION

Patterns of codon usage in certain coliphages are adapted to expression in Escherichia coli. Bacteriophage T4 may be an exception to test the rule, as it produces eight tRNAs with specificities that are otherwise rare in E. coli. A database of all known T4 DNA sequences has been compiled, comprising 174 genes and a total of 115 kb (approximately 70% of the T4 genome). Codon usage has been examined in all T4 genes; some of these are known to be expressed before, and some after, the production of phage tRNAs. The results show two different patterns of codon usage: by comparison with the early genes, the late genes exhibit a shift in preference toward those codons recognized by the phage-encoded tRNAs. The T4 tRNAs translate A-ending codons, and it is possible that the phage acquired the tRNA genes because the mutation bias of the T4 DNA polymerase forces the T4 genome toward A + T-richness.