Analysis of Paramecium tetraurelia A-51 Surface Antigen Gene Mutants Reveals Positive-Feedback Mechanisms for Maintenance of Expression and Temperature-Induced Activation

Abstract

In Paramecium tetraurelia, variable surface antigen loci show mutually exclusive expression which is controlled primarily at the transcriptional level. Clonally stable expression of a single antigen has attracted models involving self-regulation by their gene products. However, direct demonstration of self-feedback at the molecular level has been complicated due to the inability to separate the functional gene from its product as well as copy number effects associated with injected extrachromosomal DNA in the polygenomic somatic nucleus. In this study, we exploited several germ line termination and frameshift mutations in the A-51 surface antigen gene to analyze variable surface antigen expression. These mutant alleles have the same copy number as the wild-type allele and therefore eliminate possible copy number effects. The mutant alleles were not transcribed at 27°C, consistent with positive-feedback models for gene expression. However, further analysis showed that high temperatures (34°C) induced transcription of the mutant A genes even in the presence of a different antigen on the cell surface. Thus, transcription was temperature dependent. Unlike wild-type cells, transcription of the mutant A genes at high temperatures was not maintained after temperature shift back to 27°C in homozygous mutant cells. Importantly, transcription of the mutant allele was maintained at 27°C in heterozygous cells with one copy of the wild-type allele. These results indicate that expression of the wild-type gene is required to stabilize its own transcriptional state at 27°C.Heritable traits of eukaryotic cells are not always governed by DNA sequence information in the nucleus. Examples of heritable traits independent from DNA information include higher-order chromatin structures (18,20

Distinct RNA-dependent RNA polymerases are required for RNAi triggered by double-stranded RNA versus truncated transgenes in Paramecium tetraurelia

“…As mentioned above, the surface antigen gene family is controlled by unknown mechanisms ensuring that only one gene is expressed at a time. Several studies concluded that surface antigen genes are regulated at the level of transcription ( 54–57 ), although cases of post-transcriptional regulation have also been described ( 26 , 58 ). It is not known whether transcriptional and post-transcriptional control mechanisms interact.…”

Genome plasticity in Paramecium bursaria revealed by population genomics

“…Expansion of the TLDc domain-containing gene in P. bursaria may be an adaptation to such oxidative stress. Expression of Paramecium surface antigen domain-containing genes is often induced by environmental changes [73,74], but it remains unclear why Paramecium surface antigencontaining, UPF0047 domain-containing, and protein kinase domain-containing genes are highly duplicated in P. bursaria.…”

Signalling in ciliates: long- and short-range signals and molecular determinants for cellular dynamics

“…For instance, mutually exclusive expression of isoforms of variant surface antigens (vsAG, 'serotypes' recognised by antibodies) in most strains of P. tetraurelia ensures the presence of one single kind of vsAG (GPI-anchored proteins) per cell at any time (Preer, Preer & Rudman, 1981;Preer et al, 1987). This is regulated by a feedback mechanism (Gilley et al, 1990;Matsuda & Forney, 2005). When a vsAG is removed, an alternative vsAG isoform can be implanted on the cell surface -all under the control of differential transcription (Leeck & Forney, 1996).…”