A linear mitochondrial plasmid, pEM, found in certain isolates of the
basidiomycete Agaricus bitorquis, potentially encodes virus-like DNA a
nd RNA polymerases. Mitochondrial DNA from Agaricus bisporus that hybr
idizes to an internal region of pEM contains a fragmented and potentia
lly non-functional version of the carboxy terminal end of the plasmid
RNA polymerase. In this study, we present the sequence of the correspo
nding region of mitochondrial DNA from A. bitorquis. This sequence con
tained the same region of the plasmid RNA polymerase gene as was repor
ted for the mitochondrial DNA of A. bisporus, and the level of similar
ity between the A. bisporus and A. bitorquis mitochondrial sequences w
as much higher than the level of similarity between either mitochondri
al sequence and the plasmid. We propose that this plasmid RNA polymera
se-like sequence was present in the Agaricus mitochondrial genome befo
re the divergence of A. bisporus and A. bitorquis, and thus is unlikel
y to be a recent derivative of the plasmid pEM.