The integrase promoter and T′_I terminator in bacteriophasr λ and 434

The λ integrase promoter P₁ lies immediately downstream from a terminator T′₁. The P₁ promoter is activated by cII protein during lysogenization. The function of T′₁ is unknown. A deletion (trp-λΔ29), whose fusion point lies within one stem of T′₁ retains cII-activable promoter function but shows little if any transcription termination in vivo. The nucleotide sequence of P₁ is identical in λ and the related phage 434. However, the T′₁ sequences of the two phages, though located in the same position, are not detectably homologous. When restriction fragments carrying the promoter-terminator segments from each of these phages were inserted into the trp operon, both responded identically to cII activation, and both caused termination.