Mechanism of X-gal staining

[Michael S. Kay]

Greetings.  
Check out the following Nucleic Acids Research paper:
 
Witmer MR; Falcomer CM; Weiner MP; Kay MS; Begley TP; Ganem B; Scheraga HA.
     U-3'-BCIP: a chromogenic substrate for the detection of RNase A in
     recombinant DNA expression systems.
   Nucleic Acids Research, 1991 Jan 11, 19(1):1-4.

In this paper, we describe a chromogenic substrate for RNase that has an
analogous mechanism to X-gal.  The references in this paper refer to the
basic mechanism of the reaction (they are very old papers, but describe the
reaction quite clearly).  In short, it is a dimerization of the 5,5',
dibromo 4, 4' dichloroindolyl groups released upon b-gal action (or RNase
in my case).  Good luck.  

Michael S. Kay