PHAGE O29 PROTEIN P6 - A VIRAL HISTONE-LIKE PROTEIN

Phage 029 protein p6 is one of the most abundant viral proteins in 029 -infected B subtilis cells, constituting about 4% of the total cellula r proteins (about 3 x 10(6) copies/cell) at late infection. Electron m icroscopic studies showed that, in vitro, protein p6 forms heterogeneo usly-sized complexes all along 029 DNA, suggesting that protein p6 may have a role in genome packaging and organization. The low stability o f the protein p6-029 DNA complexes observed in vitro could reflect the dynamic nature of these complexes, to allow replication, transcriptio n, and encapsidation of the genome. The protein p6-DNA complex consist s of a DNA right-handed superhelix wrapped around a multimeric protein core. The DNA in this complex is strongly distorted and compacted. Pr otein p6 recognition signals have been mapped near the ends of the lin ear 029 DNA and act as nucleation sites for complex formation. Protein p6 does not recognize a specific sequence, but sequences with specifi c bendable properties that would favor the formation of the complex. P rotein p6 represses transcription from the 029 C2 early promoter, and activates initiation of 029 DNA replication that occurs from both DNA ends. The formation of nucleoprotein complexes at the origins of repli cation, as well as the specific positioning of protein p6 with respect to the DNA ends are required for the activation of replication. This suggests that the proteins involved in the initiation step of 029 DNA replication, either directly interact with protein p6, or recognize a conformational change at a specific location in the DNA. The mechanism of activation could be the local and transient unpairing of DNA at sp ecific sites, facilitated by the strong distortion of DNA conformation in the nucleoprotein complex.