Sequential excision of internal eliminated DNA sequences in the differentiating macronucleus of the hypotrichous ciliate Stylonychia lemnae.

Elimination of internal eliminated sequences (IES) during macronuclear development of the hypotrichous ciliate Stylonychia lemnae was analyzed in one cluster of macronuclear precursor DNA sequences. The results indicate that IES elimination is a highly ordered process, it starts very early during macronuclear development and has only finished immediately before DNA fragmentation takes place. It occurs in distinct steps and the IES are eliminated in a specific order, where a defined IES is only removed after complete elimination of other IES. Transfection experiments clearly demonstrate that the structure of the IES itself is not sufficient for its correct excision but other cis-acting sequences or additional structural requirements are needed for IES elimination.