A major challenge for a bacteriophage is to quickly pack lengthy (relative to its size) DNA in newly formed phage heads (capsids). This packaging involves “pressurizing” this DNA in the available space.
A good place to start reading about it is this review by Rao and Feiss: The Bacteriophage DNA Packaging Motor (Annu. Rev. Genet. 2008. 42:647–81).
Once you get the big picture, follow these proteins in phage genomes in the Phage Packaging Machinery subsystem.