Abstract
In Paramecium tetraurelia, stock 51, the A surface protein is coded by the wild type A(51) gene, present in micronuclei in two copies and in macronuclei in about 1500 copies. DNA processing, comprised of DNA cleavage, copy number amplification and telomere addition occurs at autogamy and conjugation when old macronuclei degrade and new macronuclei are formed from micronuclei. In this paper we characterize mutants with macronuclear A gene deletions. These mutants are notable in three respects. First, the mutants do not appear to be simple micronuclear deletions. Although genetic analysis shows that the d12 mutant d12(-1300) is homozygous for the allele A(-1300) and the mutant d12(+1) for A(+1), analysis by the polymerase chain reaction indicates that the micronucIei in these two mutants contain intact, but presumably altered, micronuclear A genes. They undergo deletion during DNA processing when new macronuclei are formed. Second, the position of the deletions in these alleles has been shown to change. The deficiency present in the d12 allele A(-1300) was originally determined to extend from position -1300 (relative to the start of translation of the A gene) to the end of the chromosome. Later, a derivative of this strain, homozygous for the d12 allele A(+1) was isolated in which the start site of the deletion was found to have moved from -1300 to +1. Third, a surprising interaction occurs in crosses between a line homozygous for the d12 allele and one homozygous for the wild-type A(51) allele. Previous work on the non-Mendelian d48 mutant (which has intact A(51) genes in its micronucleus, but has truncated A(51) genes in its macronucleus) has shown that intact A(51) alleles must be present in the old macronucleus in order for A(51) alleles to undergo proper processing. We find that d12 alleles act on A(51) alleles in heterozygotes such that intact macronuclear A genes are no longer required for proper processing of A(51). Thus, in crosses of 51 X d12 (either +1 or -1300) d12 exconjugants, as well as 51 exconjugants, give rise to clones carrying both intact A(51) and truncated d12 alleles. Remarkably the d12 alleles, which are themselves deleted during processing, are capable in the heterozygote of fostering normal processing of the A(51) allele.
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Selected References
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