DELETION END-POINT ALLELE-SPECIFICITY IN THE DEVELOPMENTALLY-REGULATED ELIMINATION OF AN INTERNAL SEQUENCE (IES) IN PARAMECIUM

Ciliated protozoa undergo thousands of site-specific DNA deletion even ts during the programmed development of micronuclear genomes to macron uclear genomes. Two deletion elements, W1 and W2, were identified in t he Paramecium primaurelia wild-type 156 strain. Here, we report the ch aracterization of both elements in wild-type strain 168 and show that they display variant deletion patterns when compared with those of str ain 156. The Wiles element is defective for deletion. The W2(168) elem ent 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 definit ion of all deletion endpoints are each controlled by cis-acting determ inant(s) rather than by strain-specific trans-acting factor(s). Sequen ce 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 e stablished consensus sequence correlates with the frequency of the cor responding deletion events. Our data implies the existence of an adapt ive convergent evolution of these Paramecium deleted DNA segment end s equences.