Figure 2.

An E. coli-based selection system for identifying zinc finger variants from large randomized libraries. (Left) A selection strain cell bearing a randomized zinc finger (white oval) that is unable to bind the target DNA subsite of interest (black box). This candidate fails to activate transcription of the weak promoter controlling HIS3 expression and therefore cells expressing this candidate fail to grow on HIS-selective medium. (Right) A library candidate bearing a particular zinc finger (one member of the randomized library) (black oval) that can bind the target DNA site. This candidate can activate HIS3 expression and therefore cells expressing this candidate grow on HIS-selective medium.