Table 3.

Selection pressures for the middle base of the finger 1 binding site of Zif268

DNA triplet	Sort	Amino acid (position 3)	Pe	Pf	σ32	(Pf − Pe)/σ32
NAN	6	Asn	0.031	0.18	0.030	5.0
		Gly	0.062	0.63	0.043	13.2
		Ala	0.062	0.09	0.043	<1
		Ser	0.094	0.09	0.052	<1
NCN	6	Glu	0.062	0.45	0.043	9.0
		Asp	0.031	0.18	0.030	5.0
		Gly	0.062	0.09	0.043	<1
		Ser	0.094	0.09	0.052	<1
NGN	6	His	0.031	0.20	0.030	5.6
		Lys	0.031	0.33	0.030	9.9
		Gly	0.062	0.50	0.043	10.1
NTN	6	Ala	0.062	0.45	0.043	9.0
		Met	0.031	0.18	0.030	5.0
		Gly	0.062	0.27	0.043	4.8

The frequencies of amino acids at position 3 on the finger 1 recognition helix are shown as their fractional representation (P_f) in clones isolated after six rounds of selection. Expected frequencies (P_e) were calculated from the number of corresponding NNS codons, assuming a completely random starting library. The standard deviation (σ) for each residue was calculated as σ = [P_e(1 − P_e)/n]^1/2, with n = 32.