Deletion of the Tetrahymena thermophila rDNA replication fork barrier region disrupts macronuclear rDNA excision and creates a fragile site in the micronuclear genome

Construction and dynamic characterization of a Tetrahymena thermophila macronuclear artificial chromosome

Artificial chromosomes were previously generated for use in bacteria, protists, yeast and human cells. A Tetrahymena thermophila artificial chromosome could serve as a versatile platform to study diverse aspects of Tetrahymena biology and beyond. Here, we placed a C3-type rDNA replication origin and telomere sequences from T. thermophila into a pNeo4 vector, producing the first T. thermophila macronuclear artificial chromosome (TtAC1). Circular or linear forms of TtAC1 can be stably transformed into both vegetative and conjugative T. thermophila cells. Linear TtAC1 was stably double in copy number under antibiotic selection, but its copy number was dropping without antibiotic selection pressure. Southern blot, Real-Time PCR and E. coli retransformation analyses together showed that TtAC1 vector did not integrate into the macronuclear genome, and was maintained as a linear or a circular chromosome in T. thermophila macronucleus under antibiotic selection. The use of TtAC1 for recombinant protein production was demonstrated by western blot analysis of a secreted 27 kDa TtsfGFP-12XHis protein. We present the first macronuclear artificial chromosome with species-specific chromosomal elements for use in T. thermophila studies and to aid broad recombinant biotechnology applications.

DNA Recombination Strategies During Antigenic Variation in the African Trypanosome

Survival of the African trypanosome in its mammalian hosts has led to the evolution of antigenic variation, a process for evasion of adaptive immunity that has independently evolved in many other viral, bacterial and eukaryotic pathogens. The essential features of trypanosome antigenic variation have been understood for many years and comprise a dense, protective Variant Surface Glycoprotein (VSG) coat, which can be changed by recombination-based and transcription-based processes that focus on telomeric VSG gene transcription sites. However, it is only recently that the scale of this process has been truly appreciated. Genome sequencing of Trypanosoma brucei has revealed a massive archive of >1000 VSG genes, the huge majority of which are functionally impaired but are used to generate far greater numbers of VSG coats through segmental gene conversion. This chapter will discuss the implications of such VSG diversity for immune evasion by antigenic variation, and will consider how this expressed diversity can arise, drawing on a growing body of work that has begun to examine the proteins and sequences through which VSG switching is catalyzed. Most studies of trypanosome antigenic variation have focused on T. brucei, the causative agent of human sleeping sickness. Other work has begun to look at antigenic variation in animal-infective trypanosomes, and we will compare the findings that are emerging, as well as consider how antigenic variation relates to the dynamics of host-trypanosome interaction.

Developmental regulation of the Tetrahymena thermophila origin recognition complex

The Tetrahymena thermophila DNA replication machinery faces unique demands due to the compartmentalization of two functionally distinct nuclei within a single cytoplasm, and complex developmental program. Here we present evidence for programmed changes in ORC and MCM abundance that are not consistent with conventional models for DNA replication. As a starting point, we show that ORC dosage is critical during the vegetative cell cycle and development. A moderate reduction in Orc1p induces genome instability in the diploid micronucleus, aberrant division of the polyploid macronucleus, and failure to generate a robust intra-S phase checkpoint response. In contrast to yeast ORC2 mutants, replication initiation is unaffected; instead, replication forks elongation is perturbed, as Mcm6p levels decline in parallel with Orc1p. Experimentally induced down-regulation of ORC and MCMs also impairs endoreplication and gene amplification, consistent with essential roles during development. Unexpectedly Orc1p and Mcm6p levels fluctuate dramatically in developing wild type conjugants, increasing for early cycles of conventional micronuclear DNA replication and macronuclear anlagen replication (endoreplication phase I, rDNA gene amplification). This increase does not reflect the DNA replication load, as much less DNA is synthesized during this developmental window compared to vegetative S phase. Furthermore, although Orc1p levels transiently increase prior to endoreplication phase II, Orc1p and Mcm6p levels decline when the replication load increases and unconventional DNA replication intermediates are produced. We propose that replication initiation is re-programmed to meet different requirements or challenges during the successive stages of Tetrahymena development.

The Origin Recognition Complex is required for site-specific replication initiation in eukaryotic chromosomes. Null mutations are lethal in yeast and metazoa, and hypomorphs induce genome instability, a hallmark of cancer. We exploited the unique biology of Tetrahymena to explore ORC's role in conventional and alternative replication programs. Modest experimental down-regulation of ORC1 induces genome instability in vegetative growing Tetrahymena, and diminishes the capacity to support developmentally regulated endoreplication and gene amplification, consistent with essential roles in all of these processes. ORC mutants fail to activate the ATR checkpoint response, and are compromised in their ability to elongate existing replication forks. Remarkably, ORC and MCM levels fluctuate in unexpected ways during wild type development. Most notably, programmed changes in ORC abundance do not reflect the impending DNA replication load. Relative to the vegetative cell cycle, ORC and MCM levels increase dramatically and are highest early in development, when the replication load is lowest. Conversely, ORC levels are lowest during genome-wide macronuclear endoreplication, when the replication load increases. Endocycling cells generate unconventional replication intermediates that distinguish them from vegetative ORC1 knockdown mutants. The collective data suggest that the dependence on ORC may be relaxed during late stages of macronuclear development.

Replication in context: dynamic regulation of DNA replication patterns in metazoans

Replication in eukaryotes initiates from discrete genomic regions according to a strict, often tissue-specific temporal programme. However, the locations of initiation events within initiation regions vary, show sequence disparity and are affected by interactions with distal elements. Increasing evidence suggests that specification of replication sites and the timing of replication are dynamic processes that are regulated by tissue-specific and developmental cues, and are responsive to epigenetic modifications. Dynamic specification of replication patterns might serve to prevent or resolve possible spatial and/or temporal conflicts between replication, transcription and chromatin assembly, and facilitate subtle or extensive changes of gene expression during differentiation and development.

TIF1 activates the intra-S-phase checkpoint response in the diploid micronucleus and amitotic polyploid macronucleus of Tetrahymena

The ribosomal DNA origin binding protein Tif1p regulates the timing of rDNA replication and is required globally for proper S-phase progression and division of the Tetrahymena thermophila macronucleus. Here, we show that Tif1p safeguards chromosomes from DNA damage in the mitotic micronucleus and amitotic macronucleus. TIF1p localization is dynamically regulated as it moves into the micro- and macronucleus during the respective S phases. TIF1 disruption mutants are hypersensitive to hydroxyurea and methylmethanesulfonate, inducers of DNA damage and intra-S-phase checkpoint arrest in all examined eukaryotes. TIF1 mutants incur double-strand breaks in the absence of exogenous genotoxic stress, destabilizing all five micronuclear chromosomes. Wild-type Tetrahymena elicits an intra-S-phase checkpoint response that is induced by hydroxyurea and suppressed by caffeine, an inhibitor of the apical checkpoint kinase ATR/MEC1. In contrast, hydroxyurea-challenged TIF1 mutants fail to arrest in S phase or exhibit caffeine-sensitive Rad51 overexpression, indicating the involvement of TIF1 in checkpoint activation. Although aberrant micro- and macronuclear division occurs in TIF1 mutants and caffeine-treated wild-type cells, TIF1p bears no similarity to ATR or its substrates. We propose that TIF1 and ATR function in the same epistatic pathway to regulate checkpoint responses in the diploid mitotic micronucleus and polyploid amitotic macronucleus.