| Positive control of late gene expression in P2- and P2-related phages is dependent upon small, phage-encoded transcriptional activators. These activators, exemplified by P2 Ogr, constitute a highly homologous family of novel prokaryotic zinc-finger transcription factors. All members of this family are functionally interchangeable, at least to some extent, and bind upstream of late promoters at an unusual site that includes an interrupted element of dyad symmetry. This binding is predicted to span three helical repeats of the major groove. The binding of these activators to DNA has been investigated using NucC, a member of the P2 Ogr family encoded by a cryptic prophage in Serratia marcescens. DNA bending by NucC was measured by a gel mobility shift assay, using fragments derived from a circular permutation vector carrying a NucC binding site. DNA determinants for NucC recognition were identified using a variety of base-specific chemical modifications and examining the outcome via protection and interference studies. The results support previous genetic studies that implicated specific nucleotides within the dyad repeat elements and indicate a previously uncharacterized motif for DNA:protein interactions. |
