Molecular and genetic analyses of the B type surface protein gene from Paramecium tetraurelia.

Abstract

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). F1 analysis of four B- mutants crossed with wild-type cells reveals heterozygous F1 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 F2 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 F2 progeny that stably maintain less than normal macronuclear amounts of the A gene and/or the B gene.

Non-Mendelian Inheritance of Macronuclear Mutations Is Gene Specific in Paramecium tetraurelia

“…We have previously determined that the d12.141 mutant inherits both the A and B genes in a Mendelian fashion. A cross with wild-type cells yields a Mendelian segregation pattern for the A and B genes in the F, generation with no detectable influence of parental cytoplasm (1:1:1:1 ratio of A' B+:A-B+:A+ B-:A-B-) (17). The d12.141 mutant carries both the tw marker and the ND mutation as genetic markers.…”

“…It is possible that gene location has a role in this regulatory mechanism. Both the A and B genes are located near the end of a macronuclear chromosome (telomere) (3,17). It is not clear whether genes in internal regions are regulated by the same system.…”

“…d12.141 was originally derived from the Mendelian A-mutant d12. It contains both micronuclear and macronuclear deletions of the A surface protein gene (13, 15) as well as a complete macronuclear deletion of the B surface protein gene (17). It contains the genetic marker twisty (tw), a morphological mutation, and a Mendelian trichocyst nondischarge (ND) (19) mutation.…”

Species-Specific Duplication of Surface Antigen Genes in Paramecium

“…Before continuing with the comparison of the seven groups, we analyzed one criterion that required additional characterization. Internal tandem repeats have been frequently described in some SAgs (51A, 51B, 51G, 156G, 168G, 156S) [14,16,[29][30][31]45] but not in others (51D, 156D, 51C) [13,27,28]. A bioinformatics prediction of these repeats is difficult as they sometimes do not match perfectly and vary in proteins.…”

“…The nomenclature of serotypes involves the strain number in association with letters for the individual SAg genes. Only 11 genes for expressed SAgs are known, three in P. primaurelia 156G/168G [14,16], 156D [28], and 156S [29], and eight in P. tetraurelia (without non-expressed isogene families) 51A [30], 51B [31], 51C [13], 51D [27], 51G [32], 51H [33], 51I, and 51J [26]. Some of these SAgs have been described as consisting of internal tandem repeats building the immunologically relevant epitopes [16]; however, these repeats are not conserved [13].…”