5-fluoro-orotic acid induces chromosome alterations in genetically manipulated strains ofCandida albicans

Mobilization of a diatom mutator-like element (MULE) transposon inactivates the uridine monophosphate synthase (UMPS) locus in Phaeodactylum tricornutum

Diatoms are photosynthetic unicellular microalgae that drive global ecological phenomena in the biosphere and are emerging as sustainable feedstock for an increasing number of industrial applications. Diatoms exhibit enormous taxonomic and genetic diversity, which often results in peculiar biochemical and biological traits. Transposable elements (TEs) represent a substantial portion of diatom genomes and have been hypothesized to exert a relevant role in enriching genetic diversity and making a core contribution to genome evolution. Here, through long-read whole-genome sequencing, we identified a mutator-like element (MULE) in the model diatom Phaeodactylum tricornutum, and we report the direct observation of its mobilization within the course of a single laboratory experiment. Under selective conditions, this TE inactivated the uridine monophosphate synthase (UMPS) gene of P. tricornutum, one of the few endogenous genetic loci currently targeted for selectable auxotrophy for functional genetics and genome-editing applications. We report the observation of a recently mobilized transposon in diatoms with unique features. These include the combined presence of a MULE transposase with zinc-finger SWIM-type domains and a diatom-specific E3 ubiquitin ligase of the zinc-finger UBR type, which are suggestive of a mobilization mechanism. Our findings provide new elements for the understanding of the role of TEs in diatom genome evolution and in the enrichment of intraspecific genetic variability.

Genomic Promoter Shuffling by Using Recyclable Cassettes

Genetic elements of interest can be introduced into the Saccharomyces cerevisiae genome via homologous recombination. A common method is to link such an element to a selectable marker gene to be integrated into the target locus. However, the marker gene in this method cannot be reused, which limits repeated manipulation of the yeast genome. More importantly, it cannot be conveniently used to integrate a promoter element. An alternative method is to utilize a counterselectable gene, such as URA3, with flanking tandem repeats. After integration, URA3 along with one copy of the repeat can be popped out via internal recombination, leaving behind one copy of the unwanted repeat. Here we describe a method of genetic element shuffling in which the tandem repeats are made of a set of promoters, so that after integration and popping out, only one copy of the promoter remains at the desired locus to function.

Haplotype mapping of a diploid non-meiotic organism using existing and induced aneuploidies

Haplotype maps (HapMaps) reveal underlying sequence variation and facilitate the study of recombination and genetic diversity. In general, HapMaps are produced by analysis of Single-Nucleotide Polymorphism (SNP) segregation in large numbers of meiotic progeny. Candida albicans, the most common human fungal pathogen, is an obligate diploid that does not appear to undergo meiosis. Thus, standard methods for haplotype mapping cannot be used. We exploited naturally occurring aneuploid strains to determine the haplotypes of the eight chromosome pairs in the C. albicans laboratory strain SC5314 and in a clinical isolate. Comparison of the maps revealed that the clinical strain had undergone a significant amount of genome rearrangement, consisting primarily of crossover or gene conversion recombination events. SNP map haplotyping revealed that insertion and activation of the UAU1 cassette in essential and non-essential genes can result in whole chromosome aneuploidy. UAU1 is often used to construct homozygous deletions of targeted genes in C. albicans; the exact mechanism (trisomy followed by chromosome loss versus gene conversion) has not been determined. UAU1 insertion into the essential ORC1 gene resulted in a large proportion of trisomic strains, while gene conversion events predominated when UAU1 was inserted into the non-essential LRO1 gene. Therefore, induced aneuploidies can be used to generate HapMaps, which are essential for analyzing genome alterations and mitotic recombination events in this clonal organism.

Candida albicans, a heterozygous diploid yeast, is the most prevalent fungal pathogen. It often acquires resistance to anti-fungal drugs via genome-altering recombination events. In many organisms, recombination events are analyzed using Haplotype Maps (HapMaps), which show the location of different alleles on each chromosomal homolog. Conventional HapMaps are constructed by following allelic markers as they segregate in meiotic progeny. Because C. albicans has not been shown to undergo meiosis, construction of a Candida HapMap has not been possible. We exploited the presence of whole chromosome aneuploidies in mitotic progeny of C. albicans to detect skewed ratios of different alleles, thereby determining the relationships between these alleles on each chromosomal homolog. This facilitated the construction of a HapMap for the most commonly used C. albicans laboratory strain. We then used this HapMap to identify all of the recombination events in a clinical isolate relative to the laboratory reference strain. Finally, we used this mapping approach to investigate the molecular mechanisms that affect the C. albicans genome when it is subjected to a common gene disruption technique. Our rapid HapMap construction method is generally applicable to any organism for which whole-chromosome aneuploidy events can be identified.

Transient marker system for iterative gene targeting of a prototrophic fungus

Auxotrophic microorganisms are often used for genetic engineering, because their biosynthetic deficiency can be complemented by the transforming DNA and allows selection for transformants that have become prototrophic. However, when complementation is obtained by ectopic expression this may lead to unpredictable side effects on the phenotype and, consequently, misinterpretation of experimental data. There are various ways to overcome the problem of auxotrophy, but the most reliable is to restore the function of the defective biosynthetic gene at the native genomic locus. This can be done by either sexual crossing or further genetic engineering. For fungal species lacking a perfect state or situations in which gene targeting is generally cumbersome we have developed a concept that allows transient disruption of pyrG. When the gene is in the disrupted state, multiple rounds of gene targeting can be performed with the strain. Once the desired genome engineering is completed, pyrG function can be rapidly returned to wild type by a simple selection scheme.