Mobile group I introns at nuclear rDNA position L2066 harbor sense and antisense homing endonuclease genes intervened by spliceosomal introns

Mobile group I introns encode homing endonucleases that confer intron mobility initiated by a double-strand break in the intron-lacking allele at the site of insertion. Nuclear ribosomal DNA of some fungi and protists contain mobile group I introns harboring His-Cys homing endonuclease genes (HEGs). An intriguing question is how protein-coding genes embedded in nuclear ribosomal DNA become expressed. To address this gap of knowledge we analyzed nuclear L2066 group I introns from myxomycetes and ascomycetes.

A total of 34 introns were investigated, including two identified mobile-type introns in myxomycetes with HEGs oriented in sense or antisense directions. Intriguingly, both HEGs are interrupted by spliceosomal introns. The intron in Didymium squamulosum, which harbors an antisense oriented HEG, was investigated in more detail. The group I intron RNA self-splices in vitro, thus generating ligated exons and full-length intron circles. The intron HEG is expressed in vivo in Didymium cells, which involves removal of a 47-nt spliceosomal intron (I-47) and 3' polyadenylation of the mRNA. The D. squamulosum HEG (lacking the I-47 intron) was over-expressed in E. coli, and the corresponding protein was purified and shown to confer endonuclease activity. The homing endonuclease was shown to cleave an intron-lacking DNA and to produce a pentanucleotide 3' overhang at the intron insertion site.

The L2066 family of nuclear group I introns all belong to the group IE subclass. The D. squamulosum L2066 intron contains major hallmarks of a true mobile group I intron by encoding a His-Cys homing endonuclease that generates a double-strand break at the DNA insertion site. We propose a potential model to explain how an antisense HEG becomes expressed from a nuclear ribosomal DNA locus.