Molecular characterization of the D surface protein gene subfamily in Paramecium primaurelia

Species-Specific Duplication of Surface Antigen Genes in Paramecium

Paramecium is a free-living ciliate that undergoes antigenic variation and still the functions of these variable surface antigen coats in this non-pathogenic ciliate remain elusive. Only a few surface antigen genes have been described, mainly in the two model species P. tetraurelia strain 51 and P. primaurelia strain 156. Given the lack of suitable sequence data to allow for phylogenetics and deeper sequence comparisons, we screened the genomes of six different Paramecium species for serotype genes and isolated 548 candidates. Our approach identified the subfamilies of the isogenes of individual serotypes that were mostly represented by intrachromosomal gene duplicates. These showed different duplication levels, and chromosome synteny suggested rather young duplication events after the emergence of the P. aurelia species complex, indicating a rapid evolution of surface antigen genes. We were able to identify the different subfamilies of the surface antigen genes with internal tandem repeats, which showed consensus motifs across species. The individual isogene families showed additional consensus motifs, indicating that the selection pressure holds individual amino acids constant in these repeats. This may be a hint of the receptor function of these antigens rather than a presentation of random epitopes, generating the variability of these surface molecules.

Variety of Serotypes of Paramecium primaurelia: Single Epitopes are Responsible for Immunological Differentiation

Paramecium primaurelia expresses three major types of surface antigens. We report here the identification of the gene for serotype S, which completes the sequence data of expressed serotypes of P. primaurelia. The complete open reading frame of surface antigen S was identified using a novel technique, based upon the presence of conservative regions in the non-coding areas of the multigene family. We were able to isolate the 7194-bp-long open reading frame from the macronuclear DNA for Serotype 156S. The corresponding mRNA was detected in the two serotype S-expressing stocks, 60 and 156, of P. primaurelia, which clarifies that both stocks are using the same S allele. Comparisons of the nucleic acid and the deduced amino-acid sequence showed high identity to surface antigen 51B of P. tetraurelia, sufficient to cause an immunological cross-reaction in vivo. Immunologically relevant epitopes in vivo were identified in the central regions of the genes, constructed of nearly perfect tandem repeats.

Molecular characterization of the SerL paralogs of Tetrahymena thermophila

In the pond ciliate Tetrahymena thermophila, expression of genes encoding variant forms of the cell surface immobilization antigen (i-ag) is regulated by environmental conditions. Multiple isoforms of the L i-ags are found on the surface of cells grown at <20 degrees C as well as on the surface of rseC mutants which express SerL genes constitutively. Five cDNAs encoding variant L i-ags of rseC were sequenced and their expression studied. Two additional SerL genes from natural isolates were sequenced. Members of the SerL family encode polypeptides with 148, 316, or 371 amino acids, and the i-ags have two, five, or six imperfect repeats, respectively, flanked by putative ER translocation and GPI addition signals. Each repeat contains six periodic cysteines, in contrast to eight or ten in other i-ags of T. thermophila. At least three of the five genes constitutively expressed in rseC mutants are differentially expressed in cells expressing other i-ags. Northern analysis and RT-PCR indicate that expression of some members of the SerL family is regulated by both transcription and mRNA stability while another member is regulated primarily by mRNA stability.

Sequence and expression of the SerJ immobilization antigen gene of Tetrahymena thermophila regulated by dominant epistasis

In ciliates, variable surface protein genes encoding the immobilization antigen (-ag) are expressed under different environmental conditions, including temperature and salt stress. These i-ags are GPI-linked and coat the entire external surface of the cell, including the cilia. In Tetrahymena thermophila-ag in natural isolates is the result of dominant epistasis masking the expression of the H i-ag ordinarily expressed at 20-36 degrees C. This report describes the expression and sequence of the Ser-ag. J is present on the cell surface up to 38 degrees C; above 38 degrees C SerSeranked by an A-rich 5' UTR and a 3' UTR containing putative mRNA destabilization motifs. The encoded J polypeptide consists of 438 amino acids and is rich in alanine, cysteine, serine and threonine. The N- and resemble signal peptide and GPI-anchor addition sites, respectively. The majority of the molecule consists of four imperfect repeats with 10 periodic cysteines per repeat in the pattern CX(6)CX(2)CX(21)CX(4)CX(13-15)CX(2)CX(18)CX(3)CX(11)CX(9-10). Although H i-ags encoded by paralogous SerH genes have 3.5 imperfect repeats with eight periodic cysteines per repeat, J nevertheless resembles H with respect to amino acid composition, codon usage, N- and C-termini, the arrangement of the cysteine periods, and regulation by mRNA stability. However, despite these similarities and epistasis, the evolutionary relationship between SerH and SerJ is unclear.

Analysis of the conserved cysteine periodicity of Paramecium variable surface antigens

The major surface antigens expressed by free-living and parasitic protozoa commonly contain repeating cysteine motifs. Despite the common occurrence of these repeats their functional significance remains largely unexplored. In this paper we investigate the conserved cysteine repeats within the variable surface antigens of Paramecium tetraurelia. We show that deletion of 2 entire repeating units or portions of repeats near the N-terminus does not prevent expression of the A51 variable surface antigen. Alteration of a single cysteine to serine residue also has no effect on A51 expression. In contrast, deletions near the C-terminus of the protein have identified a small segment within the repeats that is required for expression on the surface. The required region contains a number of conserved amino acid residues, yet site-directed mutagenesis of two residues (serine and threonine to alanine) did not prevent expression. These studies demonstrate the feasibility of using deletion analysis to identify regions critical for the expression of cysteine-rich surface antigens. The relationship of these results to the structure and expression of cysteine-rich surface proteins in other protozoa is discussed.

Timing of developmentally programmed excision and circularization of Paramecium internal eliminated sequences

Paramecium internal eliminated sequences (IESs) are short AT-rich DNA elements that are precisely eliminated from the germ line genome during development of the somatic macronucleus. They are flanked by one 5'-TA-3' dinucleotide on each side, a single copy of which remains at the donor site after excision. The timing of their excision was examined in synchronized conjugating cells by quantitative PCR. Significant amplification of the germ line genome was observed prior to IES excision, which starts 12 to 14 h after initiation of conjugation and extends over a 2- to 4-h period. Following excision, two IESs were shown to form extrachromosomal circles that can be readily detected on Southern blots of genomic DNA from cells undergoing macronuclear development. On these circular molecules, covalently joined IES ends are separated by one copy of the flanking 5'-TA-3' repeat. The similar structures of the junctions formed on the excised and donor molecules point to a central role for this dinucleotide in IES excision.