Keller A M, Le Mouël A, Caron F, Katinka M, Meyer E
Laboratoire de Génétique Moléculaire, Ecole Normale Supérieure, Paris, France.
Dev Genet. 1992;13(4):306-17. doi: 10.1002/dvg.1020130408.
The Paramecium primaurelia cell surface is covered with a high molecular weight protein called the surface antigen. Several genes encode alternative surface antigens, but only one is expressed at a time. In addition, each of these genes shows a high degree of allelic polymorphism. Paramecium primaurelia strains 156 and 168 have different alleles of the G antigen gene whose respective antigens can be distinguished in vivo using specific antibodies. An interallelic exclusion phenomenon has been previously described: 94% of the 156/168 heterozygotes express only the 156 allele of the G gene; 6% express both the 156 and the 168 alleles. The phenotype of the heterozygotes is determined at the time of macronuclear differentiation. We have investigated the molecular basis for the different heterozygous phenotypes. Both mRNAs are always produced, and the 156 mRNA is always more abundant than the 168 mRNA. The relative amounts of these messages, however, vary greatly between different heterozygotes and parallel their phenotype. Pushing the analysis further, we show that the copy number of each allele in the macronucleus correlates with the relative amounts of the mRNAs. However, allelic dosage alone is not sufficient to explain the variations of the mRNA ratio. The G antigen gene is located near a telomere in the macronucleus. We show that the distance between the 156G gene and the telomere is different in homozygotes and heterozygotes. It also varies among heterozygotes and is correlated with the mRNA ratio. Thus, we have identified two different parameters, both linked to the genome rearrangements occurring during macronuclear differentiation, that correlate with the relative expression of the two alleles. Two hypotheses concerning the influence of the telomere position on the expression of the gene are discussed.
嗜热四膜虫的细胞表面覆盖着一种称为表面抗原的高分子量蛋白质。有几个基因编码替代表面抗原，但一次只表达一个。此外，这些基因中的每一个都表现出高度的等位基因多态性。嗜热四膜虫菌株156和168具有G抗原基因的不同等位基因，其各自的抗原可以在体内使用特异性抗体进行区分。先前已经描述了等位基因排斥现象：156/168杂合子中有94%只表达G基因的156等位基因；6%同时表达156和168等位基因。杂合子的表型在大核分化时确定。我们研究了不同杂合子表型的分子基础。两种mRNA总是产生的，并且156 mRNA总是比168 mRNA更丰富。然而，这些信息的相对量在不同的杂合子之间有很大差异，并且与它们的表型平行。进一步深入分析，我们发现大核中每个等位基因的拷贝数与mRNA的相对量相关。然而，仅等位基因剂量不足以解释mRNA比例的变化。G抗原基因位于大核中的一个端粒附近。我们表明，纯合子和杂合子中156G基因与端粒之间的距离是不同的。它在杂合子之间也有所变化，并且与mRNA比例相关。因此，我们确定了两个不同的参数，它们都与大核分化过程中发生的基因组重排有关，并且与两个等位基因的相对表达相关。讨论了关于端粒位置对基因表达影响的两种假设。
