Evidence that core histone H3 is targeted to the mitochondria in Brassica oleracea

The core histone proteins H2A, H2B, H3 (histone H3) and H4 are known to form nucleosomes with nuclear DNA, but are historically considered to be absent from mitochondria. We suggest that H3 is a dual-targeted protein, found in mitochondria as well as N (nuclei). WoLF PSORT and MitoProt analyses of H3 sequences revealed mitochondrial targeting signals, and immunohistochemistry indicated mitochondrial distribution. Western blots of Brassica oleracea cv. Botrytis (cauliflower) mitochondrial extracts were positive for H3, when the primary antibody was against the conserved C-terminus. MS/MS (tandem mass spectrometry) analyses confirmed the Western blot data. Interestingly, Western blots of the same mitochondrial extracts were almost completely negative for H3 when the primary antibodies were highly specific for the N-terminal tail region of H3, suggesting that these antibodies are blocked by a modification of the tail of the H3 that occurs predominantly in the mitochondria, but not in the nucleus. Modifications of the tail of core H3 are known to help control nuclear genes. Future studies of the possible functions of mitochondrial H3 could lead to a greater understanding of the ability of a cell to synchronize nuclear and mitochondrial gene expression.