Figure 4.

SOS evolution pathways. The SOS-escape routes are summarized by focusing on four key amino-acid positions. The two SOS cysteines are marked in yellow, and loss of a cysteine changes the colour to grey. The oscillating 591 and 593 residues are also color-coded: red is L and, blue is Q. The observed frequencies of various reversions are indicated above the arrows. Both the original cultures (X3 and X4 in fig. 3A) and the subsequent cultures (K through T in fig. 3B) are included. The K487N reversion is left out of the scheme since it was only observed once (in X4) and the T719I reversion is not indicated since it was unchanged after its appearance in culture X.