The role of macronuclear DNA sequences in the permanent rescue of a non-mendelian mutation in Paramecium tetraurelia

Abstract

The paramecium tetraurelia mutant called d48 has a complete copy of the A surface protein gene in its micronuclei, but lacks the A gene in the macronucleus. Previous experiments have shown that microinjection of a plasmid containing the entire A gene or a large portion of the gene into the macronucleus of d48 rescued the cell line after formation of a new macronucleus (autogamy). Here we show that several different regions of the A gene can rescue d48, but 100% of the activity cannot be localized to a single, defined region. Inversion of a sequence contained within an A gene plasmid had no measurable effect on rescue efficiency and co-injection of two different plasmids results in enhancement of rescue activity despite the non-contiguous form of the DNA sequences. Both these results suggest that no specific product (RNA or protein) with defined end points is made from the rescuing fragment. A unique restriction site was created in the A gene and used to demonstrate that the injected DNA does not serve as a direct template for the synthesis of the new macronuclear DNA. Models to explain the action of the injected DNA are discussed.