Chromosomes of the fungal respiratory pathogen, Coccidioides immitis, were separated by contour-clamped homogeneous electric field gel electrophoresis. Twelve isolates were examined, the majority of which showed four chromosomes with a range of molecular size from 11.5 to 3.2 Mb. Three isolates (C634, C735, and L) revealed three chromosomal bands under the conditions employed for electrophoretic separation. However, in two of these isolates (C634 and C735), four chromosomes were visible on membrane transfers of pulsed-field gels after Southern hybridization between the chromosomal DNA and selected DNA probes. The probes included a conserved ribosomal gene and three previously described cDNAs isolated from C. immitis expression libraries. The L isolate was determined to have the same genome size as a typical four-chromosome isolate on the basis of microspectrophotometric comparison of fluorescence intensity of the ethidium bromide-stained nuclear DNA. The genome size of C. immitis determined by microspectrophotometry was approximately 28.2 +/- 2.6 Mb. The calculated genome size based on addition of the average molecular weights of chromosomal bands separated by contour-clamped homogeneous electric field gel electrophoresis was approximately equal to the estimate derived from the spectrophotometric analyses. This is the first report of the electrophoretic karyotype of C. immitis.
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