Functional elements of DNA upstream from the integrase operon that are conserved in bacteriophages 434 and lambda

A 1488-bp restriction fragment of bacteriophage 434 DNA contains the integrase promoter and an adjacent nucleotide sequence (t'I) resembling a Rho-independent terminator. To identify and quantitate transcription termination, DNA segments were cloned into a plasmid between the galactose promoter and assayable galactokinase gene and tested for termination. Whereas the entire fragment effectively terminated transcription, a 331-bp restriction fragment containing the t'I terminator had only weak terminator activity. Random sequential deletions of the 434 DNA segment defined a strong terminator 650-bp upstream from t'I. This proposed Rho-independent terminator called tL4 consists of a 7-bp stem and 6-nt loop followed by a uridine-rich region in the RNA. Phage lambda contains an even stronger tL4 terminator that differs in 4 nt from 434 tL4. Thus, despite some sequence divergence, terminator activity has been conserved in these phages. The 434 DNA segment was also tested for promoter activity. Rightward promoter activity (opposite to pL in the phage) was located about 200 bp to the right of tL4 and was followed by an open reading frame (ORF) capable of encoding a 91 amino acid protein. Promoter activity in the same approximate location was also found in phage lambda. Thus the rightward promoter, the tL4 and t'I terminators, and ORF-55 all are elements in this segment of the genome that are conserved for function despite sequence divergence.