Postreplication targeting of transformants by bacterial immune systems?

Bacteria are constantly challenged by foreign genetic elements such as bacteriophages and plasmids. Several defense systems provide immunity against such attackers, including restriction modification (R M) systems and clustered, regularly interspaced short palindromic repeats (CRISPRs). These systems target attacking DNA and thus antagonize natural transformation, which relies on uptake of exogenous DNA to promote acquisition of new genetic traits. It is unclear how this antagonization occurs, because transforming DNA is single stranded, and thus resistant to these immune systems. Here, we propose a simple model whereby these systems limit transformation by attack of transformed chromosomes once double strandedness is restored by chromosomal replication.