THE ECTOMYCORRHIZAL BASIDIOMYCETE HEBELOMA CIRCINANS HARBORS A LINEARPLASMID ENCODING A DNA-POLYMERASE AND RNA-POLYMERASE

Bulk DNA isolated from the ectomycorrhizal basidiomycete Hebeloma circ inans was treated with proteinase K and submitted to agarose gel elect rophoresis. In addition to high molecular weight genomic DNA, three mi nor bands were detected. The band with the highest electrophoretic mob ility (2.2 kbp) corresponds to double-stranded RNA. The two other band s, termed pHC1 and pHC2, were shown to be dsDNA molecules of 10.3 and 9.1 kbp, respectively. Treatment of the pHC elements with 3'- and 5'-s pecific exonucleases revealed a linear structure and proved that the 5 ' ends are protected from digestion; for pHC2, linearity was confirmed by restriction mapping. A 3.2 kbp HindIII fragment of pHC2 was cloned and sequenced; It contains two open reading frames encoding putative viral B type DNA and RNA polymerases. Thus, the fungus harbors a typic al linear plasmid, up to now, rarely described for basidiomycetes and hitherto unknown for mycorrhizal species.