Abstract: The full length mitochondrial small subunit ribosomal
(mt-rns) gene has been characterized for Ophiostoma novo-ulmi
subspecies americana. The gene was also characterized for
Ophiostoma ulmi and a group II intron was noted in the mt-rns gene
of O. ulmi. The insertion in the mt-rns gene is at position S952 and it
is a group IIB1 intron that encodes a double motif LAGLIDADG
homing endonuclease from an open reading frame located within a
loop of domain III. Secondary structure models for the mt-rns RNA
of O. novo-ulmi subsp. americana and O. ulmi were generated to
place the intron within the context of the ribosomal RNA. The in vivo
splicing of the O.ul-mS952 group II intron was confirmed with
reverse transcription-PCR. A survey of 182 strains of Dutch Elm
Diseases causing agents showed that the mS952 intron was absent in
what is considered to be the more aggressive species O. novo-ulmi
but present in strains of the less aggressive O. ulmi. This observation
suggests that the O.ul-mS952 intron can be used as a PCR-based
molecular marker to discriminate between O. ulmi and O. novo-ulmi
subsp. americana.