Doctoral Dissertations

Date of Award

5-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Microbiology

Major Professor

Jeffrey M. Becker

Committee Members

John Koontz, W. Stuart Riggsby, Thomas Chen

Abstract

Candida albicans is an important opportunistic pathogen in patients under treatments or with conditions, such as AIDS, that suppress the immune system. A basic understanding of the biology of this pathogen is essential in order to design an efficacious pathogen specific drug. The objectives of this research were to : (1) study the regulation of the peptide transport system using oxalysine-containing peptides as model compounds, (2) attempt to clone a C. albicans gene involved in peptide transport and (3) determine if fluid phase endocytosis is operative in the dimorphic fungus. An overview of the current literature related to the research presented here is given in Part I of this dissertation. Part II of this dissertation summarizes the results of the growth inhibitory effect on C. albicans of oxalysine, a lysine analog, and oxalysine-containing di-, tri-, tetra-, and pentapeptides. Several amino acids overcame ammonium ion repression and increased the toxicity of oxalysine-containing di- and tripeptides for C. albicans with little or no increase in toxicity of oxalysine or oxalysine-containing tetra- and pentapeptides. The results indicate that the dipeptide and tripeptide transport system(s) of C. albicans are regulated by micromolar amounts of amino acids in a similar fashion to regulation of peptide transport in S. cerevisiae and that multiple peptide transport systems differentially regulated by various nitrogen sources exist in C. albicans. Oxalysine is not toxic to mammalian cells and hence it may be possible to use an oxalysine-containing peptide to design an anticandidal drug. Part II of this dissertation describes the characterization of fluid phase endocytosis in C. albicans. Lucifer yellow (LY), an impermeable fluorescent dye used as a marker for fluid phase endocytosis, was internalized by C. albicans. LY was localized in vacuoles by a non-saturable, time-, temperature-, and energy-dependent process consistent with the characteristics of fluid phase endocytosis. Both the yeast and mold phases of this dimorphic fungus internalized LY, and growth in complex medium appeared to be required to enable cells to internalize LY. These studies may explain how some large molecules, such as defensins, toxins, and cationic proteins, enter C. albicans. Part IV of this dissertation describes attempts to clone a C. albicans peptide transport gene by functional complementation of a S. cerevisiae peptide transport mutant. The S. cerevisiae ptr2 mutant was transformed with a genomic C. albicans library in a multi copy vector and transformants with a PTR2+ phenotype were obtained. However, it was not possible to recover a C. albicans PTR2 homolog on a plasmid possibly due to integration of the C. albicans gene into the S. cerevisiae chromosome. Experimental approaches have been designed for the possible recovery of the putative C. albicans PTR2 homolog. Cloning of a C. albicans peptide transport gene should provide important information on the structure and regulation of the permease and facilitate the design of an anticandidal agent to be delivered to C. albicans via the peptide transport system.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS