Programmed DNA deletions in Tetrahymena: mechanisms and implications.

Ciliated protozoa carry out remarkable DNA rearrangements during nuclear differentiation, including precise deletion of thousands of specific DNA fragments from their somatic genomes. Recent molecular studies on DNA deletion in Tetrahymena have revealed two interesting sets of cis-acting regulatory sequences, one determines the general regions to be deleted and the other the precise end points of deletion. This information suggested a novel mechanism of DNA rearrangement and raised interesting questions regarding its evolutionary origin. It is speculated that this process might be related to the unique genetic property of ciliate macronuclei, which transcribe actively and divide without chromosome condensation.