THE FATE OF DELETED DNA PRODUCED DURING PROGRAMMED GENOMIC DELETION EVENTS IN TETRAHYMENA-THERMOPHILA

Thousands of DNA deletion events occur during macronuclear development in the ciliate Tetrahymena thermophila. In two deleted genomic region s, designated M and R, the eliminated sequences form circles that can be detected by PCR. However, the circles are not normal products of th e reaction pathway. The circular forms occur at very low levels in con jugating cells, but are stable. Sequencing analysis showed that many o f the circles (as many as 50% of those examined) reflected a precise d eletion in the M and R regions. The remaining circles were either smal ler or larger and contained varying lengths of sequences derived from the chromosomal DNA surrounding the eliminated region. The chromosomal junctions left behind after deletion were more precise, although dele tions in either the M or R regions can generate any of several alterna tive junctions (1), Some new chromosomal junctions were detected in th e present study, The results suggest that the deleted segment is relea sed as a linear DNA species that is degraded rapidly. This species is only rarely converted to the stable circles we detect. The deletion me chanism is different from those proposed for deletion events in hypotr ichous ciliates (2-4), and does not reflect a conservative site-specif ic recombination process such as that promoted by the bacteriophage la mbda integrase (5).