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Chapter 20 : From Acetylase to β-Galactosidase
Enzymes involved in the degradation of substrates similar in structure seemed likely to derive from the same precursor. For this reason Monod decided to study the enzymes involved in the degradation of two similar disaccharides: lactose and maltose. As a result, two communications were presented on July 12, 1948, at the French Academy of Sciences. One, signed by J. Monod, A. M. Torriani, and J. Gribetz, described the occurrence, in Escherichia coli, of lactase, only present in lactose-grown cells. In the other, signed by J. Monod and A. M. Torriani, was reported the existence of an "amylomaltase," present exclusively in maltose-grown cells. Because the "lactase" was in fact a β-galactosidase of broad specificity, it became possible to synthesize a chromogenic substrate for this enzyme, and therefore to render its assay both very simple and very sensitive, while the assay of amylomaltase still required the use of the cumbersome Warburg apparatus. The corresponding mutations generally mapped in the vicinity of the amylomaltase-phosphorylase operon. Other mutations, mapping in this second region, led to the most incredible phenotypes. The unwarranted complexity of the two most well-known positively regulated systems made Monod, and many others, suspicious that something basic could be wrong. Still, over the years the author could not escape imagining what would have happened if Monod had focused on amylomaltase, rather than on β-galactosidase.