Strain d48 of Paramecium tetraurelia contains the A i-antigen gene in the micronucleus, but the gene is lost when micronuclear products develop into the macronucleus. It has recently been shown that when injected into d48, macronucleoplasm from the wild type transforms d48 cells to wild type. It is shown here that wild-type cytoplasm can also bring about transformation, with a marked stage-specific sensitivity for both donor and recipient. It was also found that a plasmid containing the cloned A gene could transform d48 to wild type. Injection of nucleoplasm from animals in the vegetative stage of the cell cycle into the cytoplasm of recipients at various stages of autogamy caused high-frequency transformation of cells able to express the A serotype both before and after the next autogamy. Injection of nucleoplasm into vegetative macronuclei produced over 70% transformants able to express the A serotype after the next autogamy. The ability of nucleoplasm to transform was acquired at the second cell cycle after autogamy and was maintained throughout the vegetative stage. When cytoplasm was obtained from donors during autogamy and injected into the cytoplasm of recipients 1 to 2 h after the sensitive period, quite high frequencies of stable revertants were found when tested both before and after the next autogamy. Cells that were injected into the macronucleus with the cloned A plasmid expressed the A serotype after five fissions in over 20% of the lines and maintained this ability through successive fissions; all transformants except one stably expressed the A serotype even after the next autogamy.Paramecium cells have serotypes resulting from the expression of one of several specific surface proteins called immobilization antigens (i-antigens). Each cell usually exhibits one serotype, which is determined by a combination of genetic and environmental conditions. In stock 51 of Paramecium tetraurelia, 11 serotypes, A, B, C, D, E, G, H, I, J, N, and Q, have been observed (for reviews, see references 4 and 13). It appears that each i-antigen is coded by a distinct genetic locus (14). Control of the genes specifying the i-antigen occurs at the level of transcription or RNA processing.P. aurelia complex cells contain two kinds of nuclei: normally two germinal micronuclei, which are involved in sexual activity, and a somatic macronucleus, which is highly polygenomic and controls vegetative activity. Epstein and Forney (3) isolated X-ray-induced mutations affecting expression of the A i-antigen in P. tetraurelia. Strain d48, which cannot express the A antigen, does not contain the A gene in its macronucleus but contains a complete copy of the gene in its micronucleus. Genetic analysis of this mutant indicates that the cytoplasm affects the development of new macronuclei from the fusion micronucleus at conjugation and autogamy; the A gene is lost during the formation of the macronucleus in the cytoplasm of mutant d48 but not in the wild-type cytoplasm.Harumoto (7) demonstrated that transfer of macronucleoplasm from wild-...
Mutant strain d48 and d l 2 cannot express serotype A. In d48, the A i-antigen gene is present in the micronucleus, but not in the macronucleus. It has recently been shown that d12 contains the A gene in its micronucleus, but its macronucleus lacks the gene. Micronuclear transplantations into enucleated cells were performed to analyze those mutants. Reciprocal transplantation between wild type and d48 confirmed that d48 contains the A gene in the micronucleus and its cytoplasm is defective. Wild type 5 1 enucleated cells into which were transplanted d 12 micronuclei could not express A. Amicronucleate d 12 cells into which were transplanted normal micronuclei from 5 1 or d48 showed no expression of A. These results show that even if the micronucleus of d12 contains the A gene, it must be abnormal, and its cytoplasm is also defective the same as d48. Genetic analysis showed that heterozygote of d12 and wild type 5 1 or d48 caused a cure of the cytoplasmic defect of d48 and d12 during the development of macronuclei.
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