IDENTIFICATION OF 3 UBIQUITIN GENES OF THE RICE BLAST FUNGUS MAGNAPORTHE-GRISEA, ONE OF WHICH IS HIGHLY EXPRESSED DURING INITIAL-STAGES OF PLANT COLONIZATION

Differential cDNA screening was used to identify genes expressed durin g the colonisation of rice leaves by the pathogenic fungus Magnaporthe grisea. This led to the identification of a gene, called UEP1, which encodes a ubiquitin extension protein. UEP1 was highly expressed 48 h after initial fungal infection of rice leaves when M. grisea is prolif erating in the leaf epidermis but not yet causing disease symptoms. UE P1 appeared to be downregulated after this time despite further extens ive growth of the fungus throughout the leaf tissue. To investigate th e potential role of ubiquitin in fungal pathogenesis we subsequently i solated UEP3 and PUB4, encoding a second ubiquitin extension protein a nd a polyubiquitin respectively. UEP1 was expressed abundantly during active growth of M. grisea in axenic culture but was down-regulated by starvation-stress. UEP3 showed a similar pattern of expression to UEP 1 during the growth of M. grisea in culture and after environmental st ress, but was not highly expressed during plant colonisation. PUB4 was highly expressed after environmental stress, but was not highly expre ssed during plant colonisation. UEP1 was found to be present in a much -higher copy number per haploid genome compared to UEP3 and PUB4. The restricted high-level expression of UEP1 suggests that M. grisea under goes rapid ribosomal biogenesis and protein turnover during initial pl ant-tissue colonisation, which is regulated by a specific UEP1-encoded component of the M. grisea ubiquitin gene family.