Characterization of a single group I intron in the 18S rRNA gene of the pathogenic fungusHistoplasma capsulatum

Abstract

A 425-bp insertion in Histoplasma capsulatum strain G186B, denoted as Hc.SSU.1, was identified as a group I intron, based on the presence of the conserved sequence elements P, Q, R and S and a predicted secondary structure consistent for group I introns. The Hc. SSU.1 sequence from strain G186B was identical to strain G184B but differed from strain FLs1 by five nucleotides. Hc.SSU.1 was most similar to the group I intron from the black mould Exophiala castellanii. Southern blot analysis suggests that the intron is not dispersed in the genome and that most, if not all 18S rRNA genes harbour the intron. Northern blots demonstrated absence of the intron from mature 18S rRNA. A Hc.SSU.1-specific PCR assay detected the intron in six of 37 isolates of Histoplasma. Hc.SSU.1-containing strains exhibited no significant differences in antimicrobial susceptibilities when compared to isolates not containing Hc.SSU.1. This investigation demonstrates the existence of group I intron sequences in the H. capsulatum genome and its evolutionary relationship among other group I intron sequences.