Oligomeric structures of the phage O29 histone-like protein p6

Protein p6 of Bacillus subtilis phage empty set29 has been described as a h istonelike protein, playing a role in genome organization and compaction, o n the basis of its high intracellular abundance, its pleiotropic effect, an d its ability to bind and highly compact the whole empty set29 DNA in vitro . Protein p6 forms large multimeric nucleoprotein complexes in which a righ t-handed superhelical DNA wraps toroidally around the protein core. Analyti cal ultracentrifugation analysis, at the concentration estimated in vivo (a t least 1 mM), showed that protein p6 self-associates into elongated oligom ers, suggesting that, in the absence of DNA, the protein could form a scaff old for DNA binding. In this work we have studied the structure of these ol igomers by transmission electron microscopy and image processing. The resul ts show that protein p6 aggregates into crooked-shaped oligomers, compatibl e with a helical structure. The oligomers could interact head-to-tail to fo rm doughnut-shaped structures or they could grow into right-handed double-h elical filaments by a nucleation-dependent polymerization process. The dime nsions of the crooked-shaped structures are in agreement with that of the D NA in the nucleoprotein complex previously described. We propose that the c rooked-shaped structures could act as a scaffold imposing the right-handed path followed by the DNA, and thus it could be considered a non-transient D NA chaperone. (C) 1999 Academic Press.