Dear Horst et al.,
you mention "problems with FoA selection" that might perhaps be due
to the low (4.5) pH of your plates. It wasn't clear from your posting
which kind of problems - did even URA+ grow on your FoA plates, or
did you end up with no colonies even from an ura3 strain? Your concern
about pH suggests rather the later - and there you don't have to worry
If you grow yeast in a non-buffered medium, they will acidify it themselves.
In fact, yeast doesn't like neutral pH; normal pH of beer, for instance, is
between 4.5 and 5. So, whatsoever the reason for your problems is,
it is probably not pH.