Deletion endpoint allele-specificity in the developmentally regulated elimination of an internal sequence (IES) in Paramecium

Ciliated protozoa undergo thousands of site-specific DNA deletion events during the programmed development of micronuclear genomes to macronuclear genomes. Two deletion elements, W1 and W2, were identified in the Paramecium primaurelia wild-type 156 strain. Here, we report the characterization of both elements in wild-type strain 168 and show that they display variant deletion patterns when compared with those of strain 156. The Wiles element is defective for deletion. The W2(168) element is excised utilizing two alternative boundaries on one side, both are different from the boundary utilized to excise the W2(156) element. By crossing the 156 and 168 strains, we demonstrate that the definition of all deletion endpoints are each controlled by cis-acting determinant(s) rather than by strain-specific trans-acting factor(s). Sequence comparison of all deleted DNA segments indicates that the 5'-TA-3' terminal sequence is strictly required at their ends. Furthermore the identity of the first eight base pairs of these ends to a previously established consensus sequence correlates with the frequency of the corresponding deletion events. Our data implies the existence of an adaptive convergent evolution of these Paramecium deleted DNA segment end sequences.