Specific Mg binding to AT-rich regions of chromatin in the evolution of eukaryotes

At SIMS XIV, we reported SIMS evidence of specific Mg²⁺ binding to the AT-rich regions of human metaphase chromosomes represented by G-bands. Subsequent Mg²⁺-depletion experiments supported a direct role for Mg²⁺ in promoting and maintaining the higher order chromatin structure originating G-bands, possibly due to both Mg²⁺-DNA and Mg²⁺-protein interactions. An in-depth study, reported elsewhere, implicated also Ca²⁺ in the maintenance of chromatin ultrastructure in the scaffold of mammalian chromosomes, in association with topoisomerase II.We examine here the association of Mg²⁺ with AT-rich regions of chromatin in the chromosomes of the Indian muntjac deer (IMD), leading to conclusions similar to the above. To answer the question whether the presumed divalent cation role in the chromosomes of advanced eukaryotes had an evolutionary history to be traced back to earlier evolutionary stages, we have SIMS-mapped Ca²⁺ and Mg²⁺ in BrdU-labeled polytene chromosomes from the salivary gland of the Dipteran Drosophila melanogaster. Striking Ca²⁺ and Mg²⁺ SIMS banding patterns correlating with those of the Br label (a thymidine analogue) implicate unequivocally a close association of both these cations with the AT-rich regions of DNA for these primitive eukaryotes.