Group-I intron family in the nuclear ribosomal RNA small subunit genes of Cenococcum geophilum isolates

A family of optional group-I introns was found near the 3' end of the nuclear small subunit rRNA genes in 61 out of 70 isolates of the deuteromycete mycorrhizal fungus Cenococcum geophilum. DNA sequence polymorphisms among the introns (termed CgSSU introns) from ten of the isolates were studied. The sequences, ranging in size from 488 to 514 nucleotides, were from 93.2% to 99.6% similar to each other. Mutations were less common in predicted base-paired regions (33% of all mutations) than in freestanding regions (67%). The introns were self-spliced in vitro and were closest to subgroup IC1 according to sequence and predicted secondary structure. Group-I intron pairing regions P1 through P10, including core regions P, Q, R and S, were present in all ten CgSSU introns studied. No lengthy open reading frames were found in any of the introns, indicating that the introns do not encode a protein, and therefore may not be mobile. It is likely that a single intron entered a progenote of C. geophilum and changed as the species evolved.