Fluorescent labelling of histone H3: effect on histone-histone interaction and core particle assembly

Histone H2B (and H2A) ubiquitination allows normal histone octamer and core particle reconstitution

146 bp core particles were assembled from reconstituted hybrid histone octamers where either histone H2A or H2B were replaced by their ubiquitinated counterparts uH2A and uH2B. No difference in the structure of the core particles was evident upon DNase 1 digestion suggesting that ubiquitination of these histones was not a barrier to normal core particle formation.

A novel nucleosome assembly procedure (with a little help from pectin)

The acidic polysaccharide pectin (alpha-1,4-polygalacturonic acid) has been introduced as a nucleosome assembly facilitator as a substitute for polyglutamic acid. The pectin-assembled nucleosomes were indistinguishable from polyglutamic acid-assembled nucleosomes by thermal denaturation and DNAse I digestion. Pectin had two major advantages over polyglutamic acid-the yield of assembled cores was approximately 50% higher and the pectin could be easily removed after completion of the reassembly procedure by dialysis following pectinase cleavage.

Preparation of fluorescently labelled hybrid histone octamers

Labelling hybrid histone octamers (the Cys variant of histone H4 replaced histone H4 in the chicken erythrocyte octamer) with the fluorescent probe 5-(2(iodoacetyl)aminoethyl)aminonapthalene- 1-sulfonic acid, IAEDANS, resulted in significant non-specific incorporation of label. Fluorescently labelled hybrid histone octamers were prepared by reconstitution methodology after labelling the isolated histone Cys-H4 and separation of specifically and non-specifically labelled histone. Core particles prepared from these octamers have identical thermal denaturation to unlabelled core particles demonstrating that the incorporation of a fluorescent probe at this site has no overall effect on either histone-histone or histone-DNA interactions. DNase 1 digestion of 32P end-labelled fluorescent core particles yielded the anticipated asymmetric cutting pattern with a 10 bp interval between fragments. Comparison of the cutting pattern with those previously obtained in these laboratories for both polyglutamic acid reconstituted and 'native' core particles demonstrated that fluorescent core particles had an enhanced susceptibility to digestion at site 7.