Ion etching bacteriophage T4: support for a spiral-fold model of packaged DNA.

Ion etching of bacteriophage T4 erodes virus components progressively from the outside to the inside while preserving the overall structure. The terminal portion of the T4 DNA molecule packaged can be specifically radiolabeled and was found to be eroded more rapidly than the remainder of the DNA. This strongly suggests that the first DNA to enter the prohead is condensed in the center of the capsid and is therefore shielded from the ion beam by the surrounding last packaged DNA. The results support a "spiral-fold" model for the arrangement of DNA within the icosahedral bacteriophage head. According to this model, phage T4 DNA strands run parallel to the long axis of the phage, with sharp (180 degrees) bends at the top and bottom of the capsid. The folds themselves are arranged radially about the long axis of the head in spirally organized shells.