A recent PLoS One article “A Genetic Code Alteration Is a Phenotype Diversity Generator in the Human Pathogen Candida albicans” finds some pretty dramatic changes in gene expression and phenotypes by replacing the tRNAs for CUG back to Leucine (Leu; in the standard genetic code) from their meaning of Serine (Ser) in these Candida species. The CUG codon transition in some Candida spp has been of interest since it is an example of a recent change in the genetic code and provides a comparative system to study the mechanism and genome changes of how a genetic code shift is manifested.
A recent paper “Targeted gene deletion in Candida parapsilosis demonstrates the role of secreted lipase in virulence”, from the Nosanchuk lab at Yeshiva University, shows the role of secreted lipases in virulence of this pathogen. C. parapsilosis is second only to the evolutionarily closely related commensal Candida albicans as worldwide cause of invasive candidiasis. This paper demonstrates a knockout system using selectable marker which confers resistance to the drug Nourseothricin. The authors sought to delete the adjacent and convergently-transcribed lipase genes CpLIP1 and CpLIP2 and characterize the phenotype of the lipase deficient mutants as blood-borne C. parapsilosis infections are in a lipid rich environment.
Through a series of experiments testing growth in rich media, media with olive-oil, and in infection models they showed that the importance of lipase activity. The knockout strain was unable to grow efficiently on YNB media+olive oil indicating that these two genes are the only ones capable of lipase activity. The murine infection experiments indicated that the knockout could be cleared in 4 days while the WT and reconstituted were cleared in 7. The authors acknowledge some limitations in the infection model in that it does not fully recapitulate an invasive candidiasis because mice were infected intravenously so the role of endothelial cell invasion was tested in vivo.
This is not the first paper on targeted gene knockouts in this fungus. A paper from earlier this summer, “Development of a gene knockout system in Candida parapsilosis reveals a conserved role for BCR1 in biofilm formation”, from Geraldine Butler’s group at University College group, who work on both evolutionary and pathogenesis questions in Candida species, developed a knockout system using the same drug marker. The Butler laboratory also showed that the C. parapsilosis MAT locus, part of the sexual reproduction machinery of fungi, has degraded, consistent with the observed asexuality of these species.
The improving genetic tools for targeted disruption of loci in additional species is permitting experiments that get at the heart of what makes some fungi pathogenic. With the genome sequence of many of the relatives of the pathogens we can systematically dissect what genetic differences have a role in virulence. It will be interesting to reconstruct whether the ancestor of many of these Candida spp always had the potential for virulence or if it co-evolved with its human or other mammalian commensal lifestyle.
Several more fungi are on the docket for sequencing at JGI through their community sequencing program. This includes
- The Dothideomycete leaf streak disease causing fungus Mycosphaerella fijiensis
- Soybean rust Phakopsora pachyrhizi
- The Basidiomycete and jelly fungus Tremella mesenterica proposed by Joe Heitman for use as outgroup to the human pathogen Cryptococcus
- The plant pathogen Cochliobolus heterostrophus proposed by Gillian Turgeon which ironically was already sequenced at the now closed Syngenta Torrey Mesa Research Institute (i.e. this paper on NRPS which used the genome)
- The Sordariale Thielavia terrestris proposed by Novoenzymes presumably for potential in producing novel cellulases as part of biofuel production research.
- The Sordariale and Chestnut blight fungus Cryphonectria parasitica
- EST sequencing for Aspergillus terreus proposed by Scott Baker at PNNL
- Scott is also helping lead a projects to sequence Piromyces and Orpinomyces both early branching Neocallimastigomycota fungi that live in the rumen (which I am probably a little too excited about). Apparently the high A-T content is causing problems in the sequencing phase.
- Agaricus bisporus, sadly the only mushroom some people ever eat (canned and put on pizza or from canned soup), proposed by Mike Challen is also slated to be sequencing in 2008. Did Campell’s already sequence it anyways? We got to see them in their non-native habitat on a field trip in the fall (more pictures!).
- The Basidiomycete EM fungus Paxillus involutus proposed by Anders Tunlid will complement ongoing work in plant-fungal association work.
- Heterobasidion annosu, a basidiomycete fungal pathogen of conifers.
- Three Neurospora genomes proposed by our lab
- The oyster mushroom Pleurotus ostreatus
- The amphibian pathogen Batrachochytrium dendrobatidis that I’m working on with collaborators at Berkeley and the Broad Institute (which sequenced another strain)
- Trichoderma actrovirdi (which doesn’t appear to have any sequence in GenBank) is reportedly in production (bottom of the page).
This complements an ever growing list of fungal genome sequences which is probably topping 80+ now not including the several dozen strains of Saccharomyces that are being sequenced at Sanger Centre and a separately funded NIH project to be sequenced at WashU.
A recent paper I found interesting (and I am sure was interesting to Dr Logsdon) about Multiple losses of sex within a single genus of Microsporidia. In the paper Joseph Ironside describes multiple instances of loss of sex within the Nosema/Vairimorpha group testing the hypothesis that the ancestral lineage was asexual. The group of species are undergoing rapid evolution where changes in lifestyle/lifecycle can occur even among very closely related lineages. In order to do test a formal hypothesis about whether the ancestor was asexual or sexual this the author had to improve the resolution of the phylogenetic relationships of these species and deal with some technical problems due to mutational biases in the rDNA sequences. The result he found was that the ancestral lineage was sexual and that asexuality arose multiple times among these species. He also provides a caution:
“The rapid evolution of microsporidian life cycles indicated by this study also suggests that even closely related microsporidia cannot be assumed to have similar life cycles and the life cycle of each newly discovered species must therefore be completely described.”
Something one has to be careful about in comparative studies of these species.
I’m including a recapping as many of the talks as I remember. There were 6 concurrent sessions each afternoon so you have to miss a lot of talks. The conference was bursting at the seams as it was- at least 140 people had to be turned away beyond the 750 who attended.
If there was any theme in the conference it was “Hey we are all using these genome sequences we’ve been talking about getting”. I only found the overview talks that solely describe the genome solely a little dry as compared to those more focused on particular questions. I guess my genome palate is becoming refined.