galactose metabolism

[APmicro]

plz help....i need this urgently......i have isolated a mutant cell of yeast that cannot take part in a particular step of galactose metabolism...in order to identify the gene responsible for it.....i have transformed the cell with plasmid vectors containing fragments of genomes of wild type yeasts.....after growing in a medium containing only galactose as a carbon source...i find colonies......thus these cells have been able to take up gal from the medium....but my question is dis....how will i know the cells could grow because of the plasmid transformed and not due to spontaneous reversion or a second site mutation.....????