Chromosomal organization of Histoplasma capsulatum

Abstract

Large-DNA electrophoresis of strains of Histoplasma capsulatum has revealed up to seven chromosomes, with marked variation in the electrophoretic mobility of the chromosomes among different strains (Steele et al., 1989, 1991). The size of the chromosomal bands is in the megabase range, extending from the size of the largest Saccharomyces cerevisiae chromosomes to beyond that of the largest Schizosaccharomyces pombe chromosomes. Resolution of these bands required low field strength electrophoresis (0.5 — 1.0 volt/cm), with long switching times (up to 1 hour) and long gel runs (up to 1 week). In the Downs strain of H. capsulatum, six bands were resolved with contour-clamped homogeneous electric field (CHEF) gels, whereas five bands were resolved with field-inversion gel electrophoresis (FIGE). The use of chromosomal band-specific probes (most of which were random sequences) permitted the determination that at least seven chromosome-sized DNAs were present in the Downs strain. The resolution of these megabase bands by FIGE was illustrative of a double-valued relation between mobility and size described by Carle et al., (1986) in the original description of FIGE, in which larger DNAs migrate with increasingly greater mobility and thus co-migrate with much smaller DNAs. In the Downs, the four largest chromosomal DNAs migrated in this fashion. Although sizing of DNAs is problematic with FIGE, the degree of resolution of very large molecules can be facilitated with the use of FIGE by exploiting this property.