2-hybrid bait alone activation

[Michael kalchman]

Hi there,

I have a found a wierd occurence of my YTH bait plasmid:  When I do a lacZ
filter assay on yeast harbouring my bait alone plasmid from YPAD plates,
my bait plasmid turns blue alone.  However, when this same clone is plated
onto SC dropout media, it FAILS to turn blue. 

Does anyone have a comment.

BTW, I am in the vector pGBT9 (-Trp marker)

Thanks,

kalch