Chapter 6, Problem 5CHI

Syn5 is a virus that infects photosynthetic bacteria belonging to the genus Synechococcus. The Syn5 virion is icosahedral (660 Å in diameter) with a short tail and an appendage called a horn. The horn is located on a vertex directly opposite the tail. Syn5 follows the typical five steps observed for most viruses when they multiply, and progeny viruses are released when the host lyses. The assembly of Syn5 virions involves several proteins, including capsid proteins, scaffolding proteins, a portal protein, a terminase, tail proteins, and horn proteins. To better understand the assembly of Syn5 virions, scientists used a type of microscopy called Zernike phase contrast electron cryotomography. This type of microscopy improves the contrast typically seen for conventional cryotomography. In their studies, the scientists observed several different developing virus particles in infected cells. The particles were: (1) 660 Å icosahedron containing DNA and having a tail; (2) 660 Å icosahedron lacking DNA, tail, and horn; (3) 590 Å spherical particle lacking DNA, tail, and horn; (4) 660 Å icosahedron containing DNA and having a tail and horn; (5) 660 Å icosahedron containing DNA but lacking tail and horn. Based on these observations the scientists suggested a pathway for assembly of Syn5 virions. What are the functions of the portal protein and terminase? Organize the particles into an assembly pathway and indicate what you think has happened at each step. (Hint: It will help if you draw the particles.)

Read the original article: Dai, W., et al. 2013. Visualizing virus assembly intermediates inside marine cyanobacteria. Nature 502:707.