The integrase promoter and T'I terminator in bacteriophages lambda and 434

The lambda integrase promoter PI lies immediately downstream from a terminator T'I. The PI promoter is activated by cII protein during lysogenization. The function of T'I is unknown. A deletion (trp-lambda 29), 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 PI is identical in lambda and the related phage 434. However, the T'I 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.