Abstract
Selfish genetic elements, such as insertion sequences and transposons are found in most genomes. Transposons are usually identifiable by their high copy number within genomes. In contrast, REP-associated tyrosine transposases (RAYTs), a recently described class of bacterial transposase, are typically present at just one copy per genome. This suggests that RAYTs no longer copy themselves and thus they no longer function as a typical transposase. Motivated by this possibility we interrogated thousands of fully sequenced bacterial genomes in order to determine patterns of RAYT diversity, their distribution across chromosomes and accessory elements, and rate of duplication. RAYTs encompass exceptional diversity and are divisible into at least five distinct groups. They possess features more similar to housekeeping genes than insertion sequences, are predominantly vertically transmitted and have persisted through evolutionary time to the point where they are now found in 24% of all species for which at least one fully sequenced genome is available. Overall, the genomic distribution of RAYTs suggests that they have been coopted by host genomes to perform a function that benefits the host cell.