Developmentally controlled telomere addition in wild-type and mutant paramecia.

We analyzed sites of macronuclear telomere addition at a single genetic locus in Paramecium tetraurelia. We showed that in homozygous wild-type cells, differential genomic processing during macronuclear development resulted in the A surface antigen gene being located 8, 13, or 26 kilobases upstream from a macronuclear telomere. We describe variable rearrangements that occurred at the telomere 8 kilobases from the A gene. A mutant (d48) that forms a telomere near the 5' end of the A gene was also analyzed. This mutant was shown to create simple terminal deletions; telomeric repeats were added directly to the truncated wild-type A gene sequence. In both the mutant and wild-type cells, the telomeric sequences (a mixture of C4A2 and C3A3 repeats) were added to various sequences within a specific 200- to 500-base-pair region rather than to a single site. No similarities were found in the primary sequences surrounding the telomere addition sites. The mutation in d48 changed the region of telomere addition at the A gene locus; this is the first example in ciliates of a mutation that affects the site of telomere addition.