Analysis of the structure and inheritance of a linear plasmid from the obligate biotrophic fungus Blumeria graminis f. sp. hordei

Abstract

A linear plasmid is widespread among isolates of the obligate biotrophic fungus Blumeria graminis f.sp. hordei (synonym Erysiphe graminis) (Bgh), the organism that causes the disease powdery mildew on barley. We cloned and sequenced the entire plasmid of 7965 bp. The plasmid contains two identical terminal inverted repeats (TIR) of 610 bp. Two ORFs are present on opposite strands, one encoding a phage-type DNA polymerase and the other a phage-type RNA polymerase. Two large transcripts of approximately 4.2 and 5.6 kb were identified in conidia, germinating conidia and Bgh -infected barley leaves, indicating that the polymerases are transcribed at most stages of the lifecycle. The transcription start sites were localised within the TIR regions, where a putative 11-bp ARS consensus sequence was also identified. To follow the sexual transmission of the plasmid we screened 27 Bgh isolates for mitochondrial polymorphisms. One polymorphism allowed us to carry out a cross between two isolates that differed in both mitochondrial genotype and presence/absence of the Bgh plasmid. The plasmid was transmitted independently of the origin of the mitochondria. No transfer of the plasmid was observed between two Bgh isolates that were co-cultivated for 1.5 years on a common susceptible barley variety. The plasmid appears to be an autonomous replicon with no phenotypic effect on Bgh.

Phylogenetic analysis of DNA and RNA polymerases from a Moniliophthora perniciosa mitochondrial plasmid reveals probable lateral gene transfer

“…Relationships between mitochondrial plasmids from different species have been estimated using DPO and RPO nucleotide and amino acid sequences. The transmission of mitochondrial plasmids during reproduction in fungi has been the subject of many studies (Giese et al, 2003), including investigations of introgression in Neurospora (Bok et al, 1999) and asexual transmission in Cryphonectria parasitica (Murrill) ME Barr (Baidyaroy et al, 2000). Other research has shown that despite the existence of somatic incompatibility barriers, plasmid transmission has also occurred by anastomosis (Giese et al, 2003).…”

“…Mitochondrial plasmids are widely distributed in filamentous fungi and exhibit some common features, such as the presence of terminal inverted repeats (TIRs) and genes encoding DNA and RNA polymerases (DPO and RPO, respectively;Kempken et al, 1992;Kempken, 1994;Cahan and Kennel, 2005). Such plasmids are found in various Ascomycota and Basidiomycota species, including several saprophytes and plant pathogens (Giese et al, 2003), particularly in the genus Neurospora (Xu et al, 1999). They are often transmitted in the same manner as mitochondria and mitochondrial DNA, and during sexual reproduction, maternal plasmids are inherited by most or all of the resulting progeny.…”