Extended Data Fig. 5: Interplay of histone-depletion, ionic strength and polyamines.

(A) Box plot of the linear stiffness of chromosomes before and after histone depletion (in PA buffer) and in 1 M salt buffer. (B) Stretch curves of a chromosome before histone depletion and after varying times of treatment with 1 M salt, recorded in 1 M salt buffer. When chromosomal mechanics were probed in 1 M salt buffer, the lengthening was even more pronounced and independent of the duration of the treatment, since at these high ionic strengths all relevant electrostatic interactions are completely screened. (C) Recorded in regular buffer without polyamines. (D) Same chromosome as (C) recorded in 1 M salt buffer. Interestingly, the behavior of histone depleted chromosomes was again dependent on polyamines. In the presence of polyamines, chromosomes would partially recover when returning from high salt to normal salt conditions (see Fig. 2d), and the lengthening clearly depended on the duration of the high salt treatment. In the absence of polyamines however, the lengthening almost remained as drastic as under high salt conditions. This might indicate that polyamine cations could be able to partially compensate the loss of the positively charged histone complexes, essentially substituting nucleosomes with polyamines.