THE 5S RIBOSOMAL-RNA AND THE RIBOSOMAL-RNA INTERGENIC SPACER OF THE 2VARIETIES OF CRYPTOCOCCUS-NEOFORMANS

The intergenic spacers (IGS) separating the 23S-like and 16S-like rDNA s of the two varieties of the human pathogenic fungus Cryptococcus neo formans were amplified, cloned and sequenced. The C. neoformans var. n eoformans IGS was 2421 nt with 5S rRNA at positions 1228-1345 3' of th e 23S-like rRNA. The C. neoformans var. gattii IGS was 2480 nt with 5S rRNA at positions 1268-1385 3' of the 23S-like rRNA. For both varieti es the 5S rDNA genes were in the same orientation as the 16S-5.8-23S e s and encode a 118 nt molecule of identical sequence. Phylogenetic com parison of C. neoformans 5S rDNA with that of other fungi placed this fungus in close relationship with other basidiomycetes including Treme lla mesenterica, Bullera alba, and Cryptococcus laurentii. A secondary structure model for the deduced 5S rRNA was constructed by comparativ e sequence analysis. Polymerase chain reaction-amplified IGS of 12 C. neoformans var. neoformans strains revealed extensive size variation r anging from 100 to 300 nt. Size variation between strains in the lengt h of the IGS may be useful for distinguishing strains. Structurally, t he IGS were characterized by the presence of occasional short direct G C-rich 19-nt repeats. Overall IGS sequence identity between the C. neo formans varieties was only 78.5%, in sharp contrast to the identical o r nearly identical sequences for the rDNA genes, and suggests rapid ev olution for IGS sequences.