Molecular and Genetic Analyses of the B Type Surface Protein Gene from Paramecium Tetraurelia

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The gene encoding the B type variable surface protein from Paramecium tetraurelia stock 51 has been cloned and sequenced. The 7,182 nucleotide open reading frame contains no introns and encodes a cysteine-rich protein that has a periodic structure including three nearly perfect tandem repeats in the central region. Interestingly, the B gene is located near a macronuclear telomere as was shown previously for two other paramecium surface protein genes. In this paper, we characterize four independent mutants with complete macronuclear deletions of the B gene. Previous analysis of different macronuclear deletion mutants of the A surface protein gene demonstrated two types of inheritance: typical Mendelian segregation (as illustrated by d12) and cytoplasmic inheritance (shown by d48). F(1) analysis of four B(-) mutants crossed with wild-type cells reveals heterozygous F(1) cell lines derived from both parental cytoplasms contain approximately the same copy number of the B gene, as expected for a recessive Mendelian mutation. Analysis of F(2) progeny from three of these four B(-) mutant crosses indicates that one of the three exhibits a Mendelian 1:1 segregation ratio of B(+) and B(-) cell lines. The other two show a preponderance of B(+) cells, but this is not correlated with the parental cytoplasmic type. In addition to having a large number of B(+) individuals, the d12.144, A(-), B(-) mutant produced some F(2) progeny that stably maintain less than normal macronuclear amounts of the A gene and/or the B gene.

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