Complex Viruses Some virions are even more complex than anything discussed so far, being composed of several parts, each with separate shapes and symmetries. The most complicated viruses in terms of structure are some of the bacterial viruses, which possess icosahedral heads plus helical tails. In bacterial viruses, such as bacteriophage some T4 of Escherichia coli (Figure 9.5b), the tail itself has a complex structure. The complete T4 tail has almost 20 different proteins, and the T4 head has several more proteins. In such complex viruses, assembly is also quite involved. For instance, in T4 the complete tail is formed subassembly, and then the tail is added as a to the DNA-containing head. Finally, tail fibers formed from another protein are added to make the mature, infectious virion.