Determination of ribosomal DNA copy number and comparison among strains of Coccidioides.

The use of PCR-based assays to detect fungi and diagnose fungal infections as well as to monitor fungal organ burden with diseases such as coccidioidomycosis is becoming more common. The target of these assays is frequently one or more of the ribosomal DNA (rDNA) gene subunits. The multicopy nature of this gene affords greater sensitivity over single-copy genes. However, there are few studies reporting the precise number of copies of the rDNA gene per genome in pathogenic fungi. Quantitative PCR was used to determine the number of copies of rDNA as well as CTS1, a single-copy gene, in samples of Coccidioides genomic DNA by the absolute quantification method. Variability of rDNA genome copy number was determined using 13 different Coccidioides isolates and was found to vary between 20 and 146 copies per genome. This suggests that detection of rDNA will likely afford an increased sensitivity of at least 20-fold over single-copy genes. However, estimation of the number of organisms present by quantification of the rDNA cannot be made prior to knowledge of each isolate’s rDNA copy number because of the strain variation.