ИСТИНА

A unique feature of the giant phage phiKZ of Pseudomonas aeruginosa is the way of packaging its genome onto a spool-like protein structure, called the inner body. Until recently, no similar structures have been detected in other members of the genus of phiKZ-like phages. Here we performed a comparative structural study of giant phages: EL, Lin68 and phiKZ, using cryo-electron microscopy, image processing, and bioinformatics methods. We obtained the first 3D reconstruction of the EL phage, consisting of a capsid (Fig. 1), helical tail and a hexagonal baseplate. A careful examination revealed that the EL capsid is 145 nm wide along its 5-fold axis, similar to the phiKZ (1). The hexagonally packed DNA strands are clearly visible in the cryo-images of the capsid (Fig. 2A). The distance between centers of separate DNA strands is 3.09±0.19 nm (Fig. 2B), which is slightly more than 2.8 nm measured in phiKZ (2). To find and to visualize the location of the inner bodies, phage particles, frozen in vitreous ice, were irradiated with increasing doses of electrons. High-energy electrons, bombarding the sample, cause obvious radiation damage to the specimen, resulting in selective boiling and protein degradation in those area that are in contact with the DNA (3, 4). According to characteristic boiling patterns, the shape and position of the inner body in EL were identified and they appear different from those for phiKZ and Lin68 (Fig. 3). Thus, the internal organization of capsids explains how the shorter DNA of the EL phage fits into a capsid, which has the same external dimensions as phiKZ and Lin68. The genome size of the giant phages correlates with the overall dimensions of the inner body, proving it to be a crucial feature for genome packing. The similarity in the structural organization of genome packaging in EL and other phiKZ-like phages indicates that EL is phylogenetically related to phiKZ-like phages, and that, despite the absence of DNA homology, EL, phiKZ, and Lin68 descend from a common ancestor.