Inverted terminal repeats are added to genes during macronuclear development in Oxytricha nova.

Abstract

In the hypotrichous ciliate O. nova all of the macronuclear DNA is in the form of low MW, gene-sized molecules with an average size of 2200 base pairs. These molecules are produced during macronuclear development by excision from micronuclear chromosomes. All, or nearly all, of the small macronuclear DNA molecules possess an inverted terminal repeat sequence consisting of 5'' C4A4 3'' repeats. The hypothesis that this terminal sequence serves as a recognition signal for excision of gene-sized molecules from chromsomes was tested. A sequence containing the C4A4 repeat was isolated and used to screen clones of micronuclear DNA for the presence of the repeat sequence. The intact repeated C4A4 sequence is not present at the ends of macronuclear sequences as they exist in the micronuclear chromosomes. The entire terminal repeat is not a recognition seuence for gene excision but must be added to the ends of gene-sized molecules during or after the excision process.