GENETIC VARIABILITY OF THE YEAST CANIDA ALBICANS

致病酵母菌白色念珠菌表现出许多多态的例子

The pathogenic yeast c. albicans exhibits numerous examples of polymorphic variations, and spontaneously produces high frequencies of mutants having abnormal colonies. The investigator previously established that alterations of colonial forms are associated with frequent single and multiple gross chromosomal rearrangements. and has suggested that the spontaneous chromosomal aberrations arising from normal strains provide a means for genetic variation in this asexual microorganism. Furthermore, related species of the genus Candida, and strains within species of C. albicans vary in their ability to assimilate some selected carbon sources. Further tests revealed that all of the spontaneous morphological mutants also had invidiously altered assimilation patterns for carbon and nitrogen sources, including the loss and/or gain of ability to grow on certain media. Most importantly, mutants acquiring the ability to assimilate certain carbon sources were selected directly, and each type was associated with a specific change in their electrophoretic karyotypes. The investigator has characterized the phenotypes an electrophoretic karyotypes of 10 Sor+ (sorbose utilizes) and 15 Ara+ (D-arbinose utilizes) mutants, and found them to comprise three major groups generally assisted with specific chromosomal rearrangements. Although the chromosomal aberrations may be secondary effects, and may not be directly responsible for the positive mutant phenotypes, the association indicates that each class of positive mutants may have been formed by different genetic alteration. The investigator will examine if the Sor+ and Ara+ mutants arise by selection of preexisting mutants or by adaptative mutagenesis in nondividing cells in response to the selective condition. The delayed appearance and the accumulation of mutant colonies suggest that at least some mutants may be arising from nondividing cells. The investigator proposes to clone the genes or DNA segments responsible for the Sor+ and Ara+ phenotypes, by complementation of a normal strain. Also, he proposes to identify the differences of DNA between the parental strain and Sor+ and Ara+ mutants, by "representational difference analysis" (RDA), a procedure based on amplifying restriction fragments by PCR and the competitive inhibition of the amplification by similar fragments, such that only the variant fragments are ultimately recovered. DNA sequences found only in Sor+ mutants, along with DNA sequences of the related normal segments may reveal the process by which positive mutants are formed. In addition, he proposes to identify preferentially expressed genes by the "differential display" procedure, which relies on PCR amplification of cDNA fragments. The three molecular approaches could result in the identification of the mutational alterations and the genes differentially activated (or silenced) in the positive mutants.