Cloning and targeted gene disruption of EXG1, encoding exo-beta 1, 3-glucanase, in the phytopathogenic fungus Cochliobolus carbonum.

The phytopathogenic fungus Cochliobolus carbonum produces an extracellular enzyme capable of degrading beta 1,3-glucan in an exolytic manner. On the basis of partial amino acid sequences of the purified enzyme, two degenerate oligonucleotides were synthesized and used as PCR primers to amplify a 1.1-kb fragment of corresponding genomic DNA. The PCR product was used to isolate the genomic copy of the gene, called EXG1. Partial sequencing of the genomic DNA confirmed that the PCR product corresponded to EXG1. A strain of the fungus specifically mutated in the EXG1 gene was constructed by homologous integration of an internal fragment of EXG1. In the mutant, enzymatic activity and the corresponding peak of UV absorption during high-pressure liquid chromatography purification were reduced by at least 98%. However, crude culture filtrates of the mutant retained 44% of the wild-type beta 1,3-glucanase activity. This residual activity was due to two additional activities which were chromatographically separable from the product of EXG1 and which were coeluted with beta 1,3-beta 1,4-glucanase activity. Growth of the EXG1 mutant was normal on sucrose and oat bran but was reduced by 65% on pure beta 1,3-glucan. The EXG1 mutant was still pathogenic to maize.