Indeed and this can lead to false positives, it is always good to check auto-activation of the bait (or use stringent conditions that rule out auto-activation).is it when the bait itsef can activate transcription without the prey?
In this case you would need to use a yeast strain that is Leu-auxotroph. You do the Y2H assay on a minimal media plate (laking Leu), so that the auxotroph can not grow. Only when there is an interaction between the bait and the prey does the promoter get activated, which should drive Leu expression giving rise to growth of the yeast.another question i want to ask is what is the differnce between doing the yeast screen on medium lacking Leu, either with glucose or with galactose? Is it because the prey protein is often induced with galactose? if there is a weak interaction, only growth shows on Leu- with galactose, whereas, in a strong interaction, both media with show growth?
thanks in advance.