Terminal-sequence conservation identifies spliceosomal introns in ascomycete 18S RNA genes

Twenty-four new insertions were obtained from seven different locations in the nuclear 18S rDNA for seven species of the lichen-forming fungal genus P hysconia. They were analyzed allowing for terminal sequence conservation by adopting a flexible approach to exact insertion site position, and they we re compared with 12 previously reported small insertion sequences from the 18S ribosomal RNA gene. Such insertions have previously been proposed to be degenerate self-splicing group I introns; however, the methodology used he re identified consensus terminal sequences characteristic of spliceosomal i ntrons. This finding is the first suggestion that multiple spliceosomal int rons occur in ribosomal genes.