Bacteriophage T7 and its close relative T3 are relatively small DNA viruses that infect Escherichia coli and a few other enteric bacteria (肠道细菌), notably Shigella. The virion (病毒颗粒) has an icosahedral (二十面的) head and a very short tail. The T7 genome is composed of a linear double-stranded DNA molecule of 39,936 bp. Gene overlap (基因重叠) occurs in the T7 genome, as do other translational strategies such as internal translational reinitiation and internal frame shifts within certain genes, all apparently geared toward maximizing genetic economy.