T7 group of phages encode a single-subunit RNA polymerase (RNAP), which specifically recognizes a set of highly conserved T7-like promoters scattered across the phage genome. The phage RNAP and its cognate promoters constitute the simple and highly efficient T7-like transcription system, which has been widely used in the laboratory. However, so far such a transcription system has only been found in the T7 group of phages.

Because of the simplicity and high efficiency, the T7-like transcription system could also be caught and utilized by other organisms during evolution. To address this, first of all, database searches can easily reveal whether there are T7-like RNAPs in bacterial genomes, and so far, there have been several T7-like prophages or prophage remnants in bacterial genomes that were found to encode a T7-like RNAP. However, none of these elements have a set of well-conserved T7-like promoters. To detect whether bacterial genomes have T7-like promoters, I then used previously built T7-like promoter models to scan more than 200 bacterial genomes. It was surprising that more than 100 T7-like promoters were found, and within these, 12 clusters of tandem T7-like promoters were located in nine enteropathogens.

Detailed comparisons of the 12 tandem promoter containing regions show that these are prophage-like islands that have a similar genome organization. Since all 12 islands have two or three tandem T7-like promoters, I named these islands T7 island. A paper about this was recently published at Nucleic Acids Res. The following is the cover of Nucleic Acids Research volume 34 issue 4.

Twelve prophage-like T7 islands have been discovered in pathogenic bacterial genomes. These islands contain two or three tandem T7-like promoters that should be activated when a bacterial cell is infected by bacteriophage T7 or a related phage. The illustration shows genetic maps for four of the islands, Ty2, BS512, E22 and ECA, which are found in the genomes of S. enterica Ty2, S. boydii BS512, E. coli E22 and E. carotovora SCRI1043 respectively. The T7-like promoters are represented by different colored bent arrows (red, T7; green, K1F; cyan, T3; magenta, unknown T7-like) and by corresponding sequence walkers. As in previously known mobile genetic elements, two of the islands, Ty2 and BS512, are adjacent to a tRNA-Gly gene (pink arrows) and have direct repeats of the 3' end of the tRNA gene (pink arrow tips). The other two islands, E22 and ECA, have different direct repeats on their ends (cyan chevron arrows). Each island encodes an integrase (blue arrows), several putative phage-related proteins (other arrows) and often several insertion sequence elements (white arrows).