Functionally important residues in P2 Cox. (a) Sequence alignment of P2 Cox and its closest sequence and structural homologs. Single alanine substitutions of P2 Cox that have been examined within our work are in bold font. Residues marked with an x above are altered in some previously identified cox mutants. (b) Level of spontaneous phage production, measured via number of plaque forming units per colony forming unit. C-6005 is a cox deficient strain. All single alanine mutations were made on the previously designed pEE720 plasmid. All data have been normalized against the pEE720 wild type and are shown on a logarithmic scale. Error bars show the standard deviation of the normalized values over at least three independent measurements. For the raw data, please see Supplementary Table S2. (c) The positions of the residues that were exchanged into alanine in this study are shown as purple sticks in the cartoon representation of P2 Cox, colored blue to red from the N- to the C-terminal. R51 is located in a disordered loop, and its location is therefore only approximate.