Forum 09/20/2007

“DNA Poised for Release in Bacteriophage 29”

Jinghua Tang, Assistant Project Scientist – Department of Chemistry and Biochemistry, University of California, San Diego, CA

The structures of mature full and empty particles of the dsDNA bacteriophage 29 were determined by electron cryo-microscopy and three-dimensional image reconstruction methods to sub-nanometer resolution.  Without imposing any symmetry, the native protein and nucleic acid structural elements in situ reveal new details about the organization of asymmetric virus particles with complex stoichiometries.  Bulk DNA in the prolate head is tightly packed in a series of eight concentric layers, the outermost of which makes specific contacts with the inner wall of the capsid.  Asymmetric interaction of the twelve phage appendages with the five-fold symmetric capsid leads to two distinct appendage conformations (up and down).  The secondary structure of the twelve-fold virus head-tail connector, revealed for the first time in situ, shows that the structural rearrangements accompany incorporation of the connector into the phage. Comparison of maps of mature and empty particles permits identification and detailed analysis of the dsDNA substructure in the tail.  From the head, the DNA follows a path along the axis of the connector, lower collar, and tail tube, making several contacts and ending with the covalently attached terminal protein at the top of the tail knob.  The high pressure (~130 atmospheres) under which the genome is packed is believed to compress the dsDNA into a 60 diameter toroid inside the confines of the connector-collar cavity.  Such a highly distorted DNA structure may in part allow it to remain pressurized inside the capsid before it is ejected into its bacterial host during infection.