Bal31 sensitivity of micronuclear sequences homologous to [formula omitted] repeats in Oxytricha nova

Abstract

The sequence similarity and functional equivalence of telomeres from macronuclear linear DNA molecules in Oxytricha and telomeric sequences of true mitotic/meiotic chromosomes suggest that the ( C 4 A 4 C 4 A 4) n ( G 4 T 4)n sequences found at macronuclear telomeres may also function as micronuclear telomeres in Oxytricha. In this study, radioactively labeled (C 4A 4) n have been hybridized to micronuclear DNA samples that have been treated with the enzyme Bal31, which has double-stranded exonuclease activity. A time course of digestion shows that approximately 50% of the micronuclear sequences that hybridize to a C 4A 4 probe disappear with mild digestion by Bal31, suggesting that these sequences are telomeric. The remainder of the hybridizing sequences are not degraded any more rapidly than the total genomic DNA. All of the C 4 A 4 G 4 T 4 sequences that can be detected by hybridization of C 4A 4 probes to Southern-blotted restriction enzyme digests of micronuclear DNA occur in regions of the genome that are highly resistant to restriction enzyme digestion and show a clustering of sites reminiscent of telomeres in other organisms. We propose that the micronuclear C 4A 4 hybridizable sequences that are Bal31 resistant may be located near the telomere and within telomere-associated repetitive sequences that are immediately internal to telomeric ( Bal31 sensitive) C 4A 4 hybridizeable sequences.