Figure 6 .

Reconstruction experiments using purR, purO, and stem deletion mutants. (A) Roughly 100 singly mutant cells (mDEL1 or purR) were plated alone without testers on lactose medium. For comparison, 10⁸ tester cells were plated alone to detect revertants in a standard reversion assay. (B) A mixture of 100 singly mutant revertant cells were genetically marked and plated together with 10⁸ tester cells on selection medium. (C) A control experiment in which 100 doubly mutant cells were plated alone on selective medium. Behavior of tester cells alone is replotted from A. (D) A mixture of 100 doubly mutant cells and 10⁸ tester cells was plated on lactose. Accumulation of colonies from tester cells alone is replotted for comparison. For all parts of the figure, cells were pregrown on minimal glycerol adenine, plated on minimal medium with lactose plus adenine, and incubated at 30° as described in Materials and Methods. Singly mutant cells plated alone or with the tester lawn were genetically marked with a Tn10 insertion near purR⁺. Strains: TT25154 (tester strain), TT26169 (purR), TT26174 (mDEL1), TT26176 (purO), TT26175 (purO mDEL1), and TT25177 (purR mDEL1).