Differential expression of the NRG1 repressor controls species‐specific regulation of chlamydospore development in Candida albicans and Candida dubliniensis

Abstract

SummaryCandida albicans and Candida dubliniensis are opportunistic fungal pathogens that are closely related but differ in their epidemiology and in some phenotypic characteristics, including certain virulence-related traits. A comparison of these two species at the molecular level could therefore provide new insights into the biology and pathogenicity of Candida . Both species share the ability to produce chlamydospores, but only C. dubliniensis forms pseudohyphae with abundant chlamydospores on Staib agar (syn. Guizotia abyssinica creatinine agar), on which C. albicans grows as a budding yeast. To understand the basis of this species-specific, differential regulation of morphogenetic development, we set out to identify C. albicans genes that repress chlamydospore formation under these conditions. A C. albicans genomic library was integrated into the C. dubliniensis genome and transformants were screened for clones in which filamentation and/or chlamydospore production on Staib agar was suppressed. This screen identified two genes, CaNRG1 and CaPDE2 , encoding a general transcriptional repressor and a high affinity cAMP phosphodiesterase, respectively. Expression of CaNRG1 in C. dubliniensis repressed pseudohyphae and chlamydospore formation, whereas expression of CaPDE2 only reduced the extent of filamentous growth but did not affect chlamydospore formation. We found that C. dubliniensis , but not C. albicans , specifically downregulates NRG1 expression on Staib medium to allow chlamydospore development. Artificial overexpression of CdNRG1 suppressed pseudohyphal growth and production of chlamydospores in C. dubliniensis .Conversely, deletion of CaNRG1 in C. albicans resulted in chlamydospore formation on Staib agar, confirming its central role in the regulation of this morphogenetic process. Our results demonstrate that differential regulation of a single gene, NRG1 , in C. albicans and C. dubliniensis is responsible for their species-specific response to environmental signals that induce chlamydospore development.

Morphogenesis in Candida albicans

“…This approach identified the transcriptional regulator Nrg1p (Table 1) as the key element that differentiated the behavior of C. albicans and C. dubliniensis. Increased expression of NRG1 in C. albicans relative to expression in C. dubliniensis blocked the ability of Staib agar to induce chlamydospore formation (119), and when an nrg1 mutant strain of C. albicans was tested for chlamydospore formation on Staib agar, the mutant strain was induced as efficiently as C. dubliniensis. This result establishes the level of expression of NRG1 as a major determinant for the production of chlamydospores under the inducing conditions established by Staib agar.…”

“…This medium induces chlamydospores in C. dubliniensis but not in C. albicans (118). Because C. albicans and C. dubliniensis have similar genomes, efforts were made to identify the elements that generated this difference in chlamydospore generation (119). A library of C. albicans sequences was introduced into C. dubliniensis and the resulting transformants screened for recombinants that were blocked in spore formation when cultured on Staib agar.…”

Differential virulence of Candida albicansandC. dubliniensis: A role for Tor1 kinase?

“…C. dubliniensis forms abundant chlamydospores on several nutrient poor media such as Staib medium, in contrast to C. albicans, which grows solely as yeast cells. 14 Staib et al showed that this enhanced chlamydospore formation by C. dubliniensis on Staib medium was due to species-specific downregulation of NRG1 transcription.…”

Comparative Genomics and the Evolution of Pathogenicity in Human Pathogenic Fungi

“…These data indicate that functional genomic approaches will be very helpful in identifying differences in gene expression and function between strains and closely related species. For example, cross-species forward genetic screens have been used successfully for whole-genome functional comparisons between C. albicans and C. dubliniensis (17,51). Analysis of transcriptional networks in C. albicans and other ascomycete species, using transcript profiling and chromatin immunoprecipitation-microarray (ChIP-chip) analysis, showed that some transcriptional pathways have been modified specifically in Candida species.…”