Screening marine fungi for plasmids and characterization of a linear mitochondrial plasmid in a Lulworthia sp.

Twenty-one marine filamentous fungi, including 18 isolates capable of growth on hexadecane, were screened for plasmids. Twenty of the isolates screened did not have plasmids, indicating that neither the ability to grow on hydrocarbons nor the lack of this ability can be correlated with the presence of plasmids. One organism, Lulworthia sp. 046, that grows on hydrocarbons, contained a plasmid approximately 14kb in size. The plasmid, designated pQB, is linear, as shown by the results of both UV-nicking experiments and digestion with exonuclease III and lambda exonuclease. The plasmid was present in DNA extracted from mitochondria, and it banded with mitochondrial DNA when nuclear and mitochondrial DNA were separated on a cesium chloride gradient. These results suggest that the plasmid is mitochondrial in origin. Southern analysis showed that the plasmid is not homologous with nuclear or mitochondrial genomic DNA, and therefore replicates autonomously.