Bacteriophages are viruses that infect bacteria. Phage DNA packaging is one process in the assembly of mature phages that is not well characterized. Elucidating the mechanism of this process would enhance our understanding of the biological significance of the machinery. A previous study suggested that the protein gp74 from bacteriophage HK97 functions as an HNH endonuclease and is required for phage DNA packaging. In this thesis the functionality was assessed for a protein from a phage that replicates under extreme conditions. Our data suggest that the endonuclease activity may not be essential for the role of gp74 in phage DNA packaging.