IS THE METAGON AN m-RNA IN PARAMECIUM AND A VIRUS IN DIDINIUM?

Abstract

The ciliate Didinium normally does not contain metagons or mu, but can acquire them by eating paramecia which contain them. They then persist and multiply in didinia (followed for 1,000 cell generations). Although such multiplication of mu in Paramecium depends on the continuous presence of an [image] gene, it occurs in Didinium even when fed paramecia lacking this gene (as well as metagons and mu). In Didinium, as in Paramecium, mu persists and multiplies only in the continuous presence of the metagon. Exposure of didinia carrying metagons to RNase is followed by permanent loss of metagons, whereas such treatment of paramecia carrying an [image] gene yields only brief transient loss of metagon activity. Metagons can be extracted from didinia, as from paramecia, in the ribosomal and RNA fractions. Detector paramecia can be infected with metagons extracted from either organism. Regardless of the source, the metagons multiply little, if at all, when infected into m paramecia and are quickly diluted out in the course of several fissions. Attempts were made to hybridize the metagon RNA with DNA extracts of [image] and m paramecia, Didinium, mu, Tetrahymena, and Aerobacter. Tests for metagon activity of the RNA released from the DNA were negative except for RNA hybridized with DNA from Paramecium. Much more activity was shown by the RNA hybridized with DNA from [image] than from m paramecia. Metagon RNA thus appears to be complementary to DNA of the [image] locus, confirming earlier results suggesting that the metagon is m-RNA of [image] genes. Failure of the metagon to hybridize with DNA from Didinium indicates that this organism has no gene comparable to the [image] genes of Paramecium. Attempts to account for all of the facts by assuming that the metagon increases only by replication or only by production from host genes meet with difficulties. The metagon appears to replicate like an RNA virus in Didinium and to arise as the m-RNA of [image] genes in Paramecium. Its possible relation to Wright''s concept of the plasmagene and to the origin of RNA tumor viruses is recognized.