IDENTIFICATION OF A GROUP-I INTRON IN THE NUCLEAR SMALL-SUBUNIT RDNA OF PSEUDOHALONECTRIA-LIGNICOLA

A 865-bp insertion was detected within the nuclear small subunit (SSU) rDNA in two isolates (intron(+)) of the freshwater ascomycete Pseudoh alonectria lignicola by the polymerase chain reaction (PCR), The intro n sequences from the two isolates were identical to each other, and th e exon sequences from the two intron(+) isolates were identical to tho se in the intron(-) isolates in the PCR-amplified region of rDNA. Reve rse transcription PCR (RT-PCR) indicated that the intron was absent in the mature rRNA. The intron sequence has all the characteristics of a group I intron, including four conserved sequence elements (P, Q, R, and S), the presence of a U at the 5' splice site of the exon, a G at the 3' splice site of the intron, a putative internal guiding sequence , and the sequence fit a secondary structure model for group I introns . Like most introns found in nuclear rDNA, this intron was located at a highly conserved region and was devoid of long open reading frames.