How to get the clone?

[Thianchai Lakornrach - SCBT - 3636209]

	I am trying to screen my transformant yeast which contain 
YEp-based shuttle vector by URA-3 auxotroph test.From the test I have got 
the positive clones but when I check their plasmids,by glass bead 
extraction and transform back into E.coli, I can't found them. Can anyone 
explain to me why not have E.coli transformant when the auxotroph 
screening is positive? And where can I get the detail of this case?
	Thank you
      T. Lakornrach
     SCI.MU. THAILAND