| Description |
The ciliated protozoan Oxytricha is an ideal organism for studying DNA breakage and rejoining, resulting in the excision of intervening segments of DNA, because they do this on a massive scale during development of the macronucleus. Two types of internal eliminated sequences or lESs are excised during macronuclear development: Large 4 kb lESs like Telomere Bearing Elements 1 (TBE1 s) are families of transposable elements found III high copy number. Small (<75 bp) lESs, also found in high copy number, differ greatly in their primary sequence. It is believed that excision of TBEI s and small lESs may involve the same mechanism. TBEls encode transposase, which may act as the excisase to remove the elements. Small lESs are too small to encode proteins, but one would expect them to contain cis-acting sequences to direct their excision. In order to identify the cis-acting sequences required for excision of small lESs, we set out to sequence a number of alleles, III two Oxytricha species, of the HMSC gene, all alleles of which contain two small lESs. An alignment of these lESs shows conserved regions, hypothesized to be important for excision. To our surprise, we found that some alleles of the gene bear precisely the same IES sequence, despite the rapid divergence of other alleles of the IES. We argue that these apparent IES-IES gene conversions may be the results of IES transposition. |
