Relative mutation rates of nucleotide i to j per site i for ex-operonic intergenic sites were calculated from singleton SNPs for the two replicatory strands.

Relative rates were derived from the following leading strand ex-operonic SNP counts, where XY represents a change from nucleotide X to Y: AG 31 GA 12 CG 0 GC 1 GT 22 TA 8 TC 21 TG 2 AC 3 CA 6 AT 11 CT 23. Nucleotide frequencies at compositional equilibrium were derived from the relative mutation rates. Leading and lagging equilibrium AT skews were calculated from equilibrium A and T frequencies. 95% bootstrap intervals are shown in parentheses.