The recent transfer of a homing endonuclease gene.
The myxomycete Didymium iridis (isolate Panama 2) contains a mobile group I intron named Dir.S956-1 after position 956 in the nuclear small subunit (SSU) rRNA gene. The intron is efficiently spread through homing by the intron-encoded homing endonuclease I-DirI. Homing endonuclease genes (HEGs) usually spread with their associated introns as a unit, but infrequently also spread independent of introns (or inteins). Clear examples of HEG mobility are however sparse. Here, we provide evidence for the transfer of a HEG into a group I intron named Dir.S956-2 that is inserted into the SSU rDNA of the Costa Rica 8 isolate of D.iridis. Similarities between intron sequences that flank the HEG and rDNA sequences that flank the intron (the homing endonuclease recognition sequence) suggest that the HEG invaded the intron during the recent evolution in a homing-like event. Dir.S956-2 is inserted into the same SSU site as Dir.S956-1. Remarkably, the two group I introns encode distantly related splicing ribozymes with phylogenetically related HEGs inserted on the opposite strands of different peripheral loop regions. The HEGs are both interrupted by small spliceosomal introns that must be removed during RNA maturation.
Haugen
Peik
P
Department of Molecular Biotechnology, RNA Research Group, Institute of Medical Biology, University of Tromsø N-9037 Tromsø, Norway.
Wikmark
Odd-Gunnar
OG
Vader
Anna
A
Coucheron
Dag H
DH
Sjøttem
Eva
E
Johansen
Steinar D
SD
D004275:DNA, Ribosomal / Q000235:genetics; D004720:Endonucleases / Q000145:classification / Q000235:genetics*; D019143:Evolution, Molecular; D007438:Introns; D016254:Mutagenesis, Insertional; D009235:Myxomycetes / Q000201:enzymology / Q000235:genetics; D010802:Phylogeny; D012326:RNA Splicing; D016337:RNA, Catalytic / Q000235:genetics; D017381:Spliceosomes / Q000378:metabolism
D004275:DNA, Ribosomal; D016337:RNA, Catalytic; D004720:Endonucleases