Figure S1. Continuous variation binding analysis (Job plot) for mithramycin A (MTA)
and mithramycin SK (MSK) binding to salmon testes DNA. The sum of the components
was kept constant at DNA (bp) + antibiotic = 40 M. The difference of absorbance at
A400nm for MTA or at A420nm for MSK is shown as a function of the mole
fraction of each antibiotic (XF). The cross-over points were obtained from linear least
squares fit to each data portion, from which we calculated: n (MTA) = 5.37 (±0.12); n
(MSK) = 5.49 (±0.13), which are in keeping with a stoichiometry of about 1 Mg2+coordinated antibiotic molecule per 5-6 base pairs of DNA observed by NMR —see the
main text for details—.