Nucleosomal core particles containing the right- and left-handed conformations of DNA were examined for their ability to support the B→Z or Z→B transition. Nucleosomes were assembled onto the B- and Z-conformations of poly[d(Gm5C)] and the B-conformation of poly[d(GC)] as previously described (1). Absorbance and circular dichroic spectroscopy indicated that the DNA on all three core particle populations could undergo the conformational B↔Z transition. Further, the right- to left-handed transition for both poly[d(Gm5C)] and poly[d(GC)] appeared to be facilitated by the DNAs association with the histone octamer. The DNA remained associated with the protein core subsequent to the transition, and electron microscopy and sedimentation velocity analysis indicated that there were no gross changes in nucleosomal structure. However, a change in the sedimentation value of the poly[d(Gm5C)] core particles was detected when the conformation of the DNA was altered from B to Z, resulting in a lower S20,w value for the Z-form particles than for the corresponding B-form particles.
An anti-Z-antibody-binding region between PM2-DNA map units 0.05 and 0.18, containing approx. 25% of the bound PM2 antibody molecules (1,2) has been sequenced. Analysis of this PM2 DNA sequence from map units 0.00 to 0.175 demonstrates that alternating purine/pyrimidine tracts capable of adopting the left-handed conformation are present within this antibody-binding region. Longer (GC)n-rich tracts are clustered together and comprise seven alternating purine/pyrimidine-rich areas (48%–84%) ranging from 19 to 142 nucleotides in length. The DNA located between these alternating purine/pyrimidine-rich areas exhibit a low level (0%–19%) of this sequence arrangement. There is a very strong correlation between the alternating purine/pyrimidine-rich areas and the anti-Z-DNA-IgG-binding sites. Nucleotides 1461–1583 of the PM2-DNA genome encode the bacteriophage capsid protein IV. One of the PM2 left-handed sites is located within this protein-coding sequence; a B-to-Z transition within this site may be involved in protein-IV gene regulation in vivo.