Development of a GUS reporter gene system for the maize pathogen Ustilago maydis

A GUS reporter gene system has been developed for the maize smut pathogen, Ustilago maydis, by making a transcriptional fusion between the coding region of the Escherichia colt gusA gene and an U. maydis hsp70-like gene promoter. This construction was transferred into U. maydis in a plasmid (pGR3) which also carried the carboxin resistance (Cbxr) gene as a selectable marker. GUS activity in these transformants appeared to be the result of expression of the gusA gene under the control of the hsp70 promoter sequences. GUS activity was visualized in colonies of transformants grown on agar plates and was quantitatively assayed in cell extracts. These results demonstrated the versatility of the GUS reporter system for studying gene expression in U. maydis. A derived plasmid (pGR4) which contained a promoterless gusA gene was also constructed. Transformants containing pGR4 did not express GUS activity. This plasmid now forms the basis of a promoterprobe vector for isolating U. maydis DNA fragments with gene promoter activity.