A Mechanistic And Functional Study Of White Opaque Phenotypic Switching In Candida Albicans

A Mechanistic and Functional Study of White-opaque Phenotypic Switching in Candida Albicans

Candida albicans is a major fungal pathogen of humans. It is a harmless commensal in healthy individuals, but it can cause serious infections in immune-compromised hosts. C. albicans undergoes a meta-stable and reversible switch between two distinct cell types known as white and opaque. The role of white-opaque switching in the biology of C. albicans was originally not well understood. We discovered an unexpected relationship between white-opaque switching and the sexual cycle of C. albicans . The mating type locus of C. albicans (MTL) encodes transcriptional regulatory proteins that regulate mating. We demonstrated that two MTL -encoded homeodomain proteins, a 1 and alpha2, work together to repress white-opaque switching in C. albicans . The observations that the MTL locus controlled both mating and white-opaque switching led us to hypothesize that opaque cells played a role in mating. Indeed, we found that opaque cells mate one million times more efficiently than do white cells. Additionally, opaque cells, but not white cells, developed specialized mating projections when exposed to mating pheromone. Thus, opaque cells are a specialized mating form of C. albicans . As white cells are generally more robust in a mammalian host than opaque cells, this strategy allows the organism to survive the rigors of life within a mammalian host, while generating a small population of mating-competent cells. The mechanism that controls white-opaque switching is not well understood. To better understand the mechanism, we investigated the regulation of white-opaque switching by a 1-alpha2. We demonstrated that a 1-alpha2 regulates white-opaque switching by destabilizing opaque cells, and we monitored gene expression during the transition from opaque to white using DNA microarrays. We used a candidate approach to identify additional regulators of white-opaque switching among genes enriched for their expression in the opaque phase. We identified two potent positive regulators of white-opaque switching: the opaque-specific transcriptional regulatory proteins Czf1 and Naf1. We also further investigated the role of Efg1, a previously identified regulator of white-opaque switching, and found that efg1/efg1 mutants were unable to bypass a 1-alpha2 repression of switching. Finally, we determined the epistatic and regulatory relationships between Efg1, Czf1 and Naf1.

Dissertation Abstracts International

Structure and Function of the Aspartic Proteinases

In September, 1990, a group of 160 scientists from 19 countries and 21 of the United States met at the Red Lion Inn in Rohnert Park, Sonoma County, California. The purpose of this meeting was to share new information from recent research on the Aspartic Proteinases. This book is a compilation of the information transferred in that forum. The Aspartic Proteinases include all those enzymes from the "fourth" class of proteolytic enzymes, the first three being the Serine, Cysteine and Metalloproteinases. Of course, all the scientists in attendance at the Sonoma Aspartic Proteinase Conference would agree that our current level of understanding of the structure and function of the Aspartic Proteinase class of enzymes is clearly first class. The reasons for this require a bit of historical perspective. The group of scientists who are engaged in study of this family of enzymes first met as a separate entity in 1976, in Norman, Oklahoma, at a meeting organized by Jordan Tang of the Oklahoma Medical Research Foundation. This was an exciting time, as the first crystal structures of some of these enzymes were described by Blundell, James and Davies. During that conference, the relationship between the two halves of the mammalian and fungal enzymes was recognized and this has provided a structural foundation for analysis of the retroviral enzymes, which came later. A book was published by Plenum Press documenting l this conference, and the current book is an update to that important work.