Functional and structural dissection of the tape measure protein of lactococcal phage TP901-1

The tail tape measure protein (TMP) of tailed bacteriophages (also called phages) dictates the tail length and facilitates DNA transit to the cell cytoplasm during infection. Here, a thorough mutational analysis of the TMP from lactococcal phage TP901-1 (TMP(TP901-1)) was undertaken. We generated 56 mutants aimed at defining TMP(TP901-1) domains that are essential for tail assembly and successful infection. Through analysis of the derived mutants, we determined that TP901-1 infectivity requires the N-terminal 154 aa residues, the C-terminal 60 residues and the first predicted hydrophobic region of TMP(TP901-1) as a minimum. Furthermore, the role of TMP(TP901-1) in tail length determination was visualized by electron microscopic imaging of TMP-deletion mutants. The inverse linear correlation between the extent of TMP(TP901-1)-encoding gene deletions and tail length of the corresponding virion provides an estimate of TMP(TP901-1) regions interacting with the connector or involved in initiator complex formation. This study represents the most thorough characterisation of a TMP from a Gram-positive host-infecting phage and provides essential advances to understanding its role in virion assembly, morphology and infection.