GROUP-I INTRONS IN 26S RIBOSOMAL-RNA GENES OF GAEUMANNOMYCES-GRAMINISAS POSSIBLE INDICATORS OF HOST-SPECIFICITY OF G. GRAMINIS VARIETIES

The 26S rRNA genes of Gaeumannomyces graminis exhibit length polymorph isms due to the presence of introns. Three group I introns have been d iscovered and their distribution appears to correlate with the three v arieties of G. graminis. No intron is found in var. graminis and Phial ophora sp. (lobed hyphopodia) which are generally not pathogenic to wh eat, oafs and barley. G.g. var. avenae isolate, 91/56 and var. tritici isolate, 90/921 have a common intron (intron AT) and a unique intron each (intron A and intron T respectively). The three introns are inser ted at different sites in the 26S rRNA genes. Intron T is self-splicin g. The sequence of intron A is more similar to the intron I of the 26S rRNA gene from human-derived Pneumocystis carinii isolate, Pc3 and of the 18S rRNA of Ustilago maydis than to the other two G. graminis gro up I introns. The catalytic core elements and stem P8 of intron T are extremely similar to corresponding regions of the group I intron in th e 26S rRNA gene of Candida albicans. Intron AT has a unique sequence. Each of the three introns appeared to have a different origin.