Abstract
The dimorphic human fungal pathogen C. albicans has broad metabolic flexibility that allows it to adapt to the nutrient conditions in different host habitats. C. albicans builds large carbohydrate stores (glycogen) at the end of exponential growth and begins consumption of stored carbohydrates when nutrients become limiting. The expression of genes required for the successful transition between host environments, including the factors controlling glycogen content, is controlled by protein kinase A signaling through the transcription factor Efg1. In addition to the inability to transition to hyphal growth, C. albicans efg1 mutants have low glycogen content and reduced long-term survival, suggesting that carbohydrate storage is required for viability during prolonged culture. To test this assumption, we constructed a glycogen-deficient C. albicans mutant and assessed its viability during extended culture. Pathways and additional genetic factors controlling C. albicans glycogen synthesis were identified through the screening of mutant libraries for strains with low glycogen content. Finally, a part of the Efg1-regulon was screened for mutants with a shortened long-term survival phenotype. We found that glycogen deficiency does not affect long-term survival, growth, metabolic flexibility or morphology of C. albicans. We conclude that glycogen is not an important contributor to C. albicans fitness.
Highlights
Microbes need to carefully manage macronutrients such as carbohydrates to survive the challenges of supporting energy metabolism in an ever-changing environment
Glycogen content and survival were analyzed in cultures of the WT SN152 and efg1/efg1 mutant strains from the Transcriptional Regulator Knockout collection [19] grown at 30 ◦ C on solid YPD
Viability and glycogen content decline over time: While glycogen in the WT strain is completely spent after 10 days, cultures retain residual viability for several days further
Summary
Microbes need to carefully manage macronutrients such as carbohydrates to survive the challenges of supporting energy metabolism in an ever-changing environment. The constant availability of carbohydrates is important for survival and growth: In this organism carbohydrates are essential for both aerobic and anaerobic energy metabolism as well as for the synthesis of a cell wall that contains a large amount of the carbohydrates glucose, mannose and chitin [1,2]. The fundamental mechanisms of yeast carbohydrate storage and their regulation have been established in the well-researched model yeast Saccharomyces cerevisiae [3]. This fungus synthesizes glycogen as the main form of carbohydrate storage when carbon, nitrogen or phosphorous become limiting [4]. There are two, functionally different glycogen synthase genes in S. cerevisiae [9], GSY1 and GSY2, both of which are controlled by the protein kinase A signaling pathway through the transcription factors Msn and
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