Figure 2.

The ade2 adeX mutation system. (A) Relevant genes in the adenine biosynthetic pathway are indicated. A red pigment accumulates in the absence of either the ADE1 or the ADE2 gene product; production of the pigment is blocked by inactivation of any gene product that functions prior to Ade1/Ade2 in the pathway. PRPP, phosphoribosyl pyrophosphate; AIR, aminoimidazole ribotide; CAIR, carboxyaminoimidazole ribotide; IMP, inosine monophosphate. (B) Following DNA damage, pure white colonies are produced only if an adeX mutation is present in both strands of duplex DNA. Bypass of damage during DNA replication results in a red-white sectored colony.