Interaction of the P1 c1 repressor with P1 DNA: Localization of repressor binding sites near the c1 gene

Abstract

The c1 repressor of phage P1 was previously shown (B. R. Baumstark and J. R. Scott, 1980, J. Mol. Biol. 140, 471–480) to bind specifically to P1 BamHI-9, a 1.4-kb fragment that is closely linked to the c1 structural gene and spans the ends of the P1 genetic map. The position of the repressor binding site(s) relative to the ends of the genetic map and the c1 gene was investigated by testing cloned fragments of EcoRl-7 and BamHI-9 for c1 expression and repressor binding. Although sequences in both BamHI-9 and the adjacent 2.7-kb EcoRl1 BamHI fragment were found to be required for the production of the c1 protein, c1 expression could be restored to the 2.7-kb fragment by the addition of a heterologous promoter (p tac). These observations are consistent with the localization of the c1 reading frame to the 2.7-kb fragment and at least part of the ci promoter region to BamHI-9. The c1 repressor was shown to bind in vitro to two distinct cloned fragments of BamHI-9 derived from the far right side of the P1 map, indicating the presence of at least two recognition sites in this region. DNA sequence analysis revealed that these two fragments share a 23-bp region of homology. A synthetic DNA containing an 11-bp sequence from this region acts as an effective competitor for repressor binding in vitro, suggesting that at least part of the sequence shared by the fragments is involved in repressor-DNA recognition.