Hi, the 4-MUG is definitely more sensitive than either X-gal or ONPG by
a factor of 10-50. The problems with 4_MUG are several-fold. First, it
diffuses rapidly once it has been cleaved so you have to look quickly on
filters. It also is brightest at high pH (8-10), so you may have to
alter this from the normal B-gal preference (pH7). Last, this cmpd does
not readily enter yeast cells. This all said, 4-MUG makes for a very
sensitive in vitro (extract) detection of B-gal. Look for newer
substrates in the Molecular Probes catalog (503) 465-8300) that are
permeable and cell-retained. Good luck.
