This family of proteins is found in Caudovirales. It is a late-transcription coactivator which interacts with the host RNA polymerase forming a part of the initiation complex [1].

1: Crystal structure of the bacteriophage T4 late-transcription coactivator gp33 with the beta-subunit flap domain of Escherichia coli RNA polymerase. Twist KA, Campbell EA, Deighan P, Nechaev S, Jain V, Geiduschek EP, Hochschild A, Darst SA; Proc Natl Acad Sci U S A. 2011;108:19961-19966. PMID:22135460