T7

T7 is a designation used in several fields, but it is most prominently associated with bacteriophage biology. Escherichia coli phage T7 is a virulent dsDNA virus that infects E. coli and is widely used as a model organism in genetics and molecular biology. The phage has a linear genome of about 40,000 base pairs and belongs to the Podoviridae family. Its life cycle is lytic; after adsorption to the host cell, early gene expression is carried out by the host RNA polymerase, followed by expression of late genes. Transcription of late genes is carried out by the T7 RNA polymerase, which is supplied by the phage itself after a short initial expression.

T7 RNA polymerase is a single-subunit enzyme encoded by gene 1 of the T7 genome. It recognizes the T7 promoter and drives high-fidelity transcription with strong processivity. The enzyme is widely used in molecular biology because it can selectively transcribe from promoters that are not recognized by host RNA polymerases, enabling controlled in vitro transcription and high-level expression in certain systems.

The T7 promoter is a short DNA sequence that specifies the transcription start site for the T7 RNA polymerase. The canonical promoter is 17 bases long, commonly represented as TAATACGACTCACTATA, with transcription initiating at the first base following the promoter. It is widely used in expression vectors and in vitro transcription kits.

Other uses of the T7 designation exist in various domains, including transportation and product naming, but in biology the T7 system remains the best characterized and most widely used reference.