(b, 3) (5,4) , using which one substitutes the substring (b,2)(4,5)(3,2) by the substring (b, 3)(5, 4).

The discovery is del:)cribed in Prescott et al. [49]. Subsequent discoveries on stichotrich macronuclear DNA are reviewed in Prescott and Rozenberg [54]. A detailed review of ciliate DNA can be found in Prescott [47]. A succinct recent review of DNA processing in stichotrichs can be found in Prescott [48]. 3 Molecular Operations for Gene Assembly We can only speculate about the significance of the extraordinary modifications in the structure of micronuclear genes in stichotrichous ciliates that have occurred during evolution.

Courtesy of Gopal Murti The genes in the micronuclear DNA of stichotrichs have extraordinary structural features not observed in the genes of any other kind of organism. The genes are interrupted by short noncoding segments of DNA called internal eliminated segments, or lESs. A few genes are interrupted by a single IES; most genes contain many lESs. 6 contains a diagram of a micronuclear gene that encodes a protein known as ,BTP. It has six lESs, which divide the gene into seven segments, known as macronuclear destined segments, or MDSs 2 IES I MD 3 IES 2 5 4 I£S 3 2 3 I£ S4 4 !