HOC and SOC are dispensable T4
capsid proteins that can be used for phage display of multiple copies of
peptides and
proteins. A bipartite phage T4
peptide library was created by displaying on tetra-
alanine linker
peptides five randomized
amino acids from the carboxyl-terminus of SOC and five randomized
amino acids from the amino terminus of HOC. The bipartite library was biopanned against the
phage T4 terminase large subunit gp17 to identify T4 gene products that may interact with the
terminase. The sequences of selected phages displayed matches to those T4 gene products previously known by genetic and biochemical criteria to interact with gp17: gp20 (portal
protein), gp32 (
single-stranded DNA binding protein), gp16 (
terminase small subunit), and gp17 (self). In addition, matches were found to gp55 (T4 late
sigma factor), gp45 (sliding clamp), gp44 (clamp loader), gp2 (
DNA end
protein), and gp23 (major
capsid protein). Abundant amino acid sequence matches were found to aa region 118-134 of gp55. Immunoprecipitation and affinity column chromatography demonstrated direct binding of gp17 and gp55; moreover, gp17 bound specifically to a column-coupled
peptide corresponding to gp55 residues 111-136. Measurements of gene 17 and other
mRNA levels in mutant-infected bacteria did not support a role of gp17-gp55 interaction in regulation of
terminase or other late gene transcription. However, whereas
DNA concatemers that accumulate in prohead and
terminase defective phage T4
infections could be packaged in vitro to approximately 10% wild-type efficiency, 55am33am defective concatemeric
DNA was packaged at least 100-fold less efficiently. Moreover, gp55 residues 111-136
peptide specifically blocked DNA packaging in vitro. These results suggest that the T4
terminase interaction with T4 late
sigma factor gp55 plays a role in DNA packaging in vivo. The gp55 interaction may function to load the
terminase onto
DNA for packaging.