Secondary lambda attachment site in the threonine operon attenuator of Escherichia coli.

We have determined the nucleotide sequence of a secondary gamma attachment (att) site which overlaps the Escherichia coli threonine (thr) operon attenuator. The secondary att site shows uninterrupted homology (8 out of 15) with the 15 base-pair "common core" sequence found in gamma and at the primary bacterial attachment site. These 8 base paired also overlap the thr operon attenuator. Comparison of the secondary att site with the flanking prophage sites shows that the crossover site for gamma integration lies within the -7 to 0 region of the core. Sequences on both sides of the core show no obvious homology with analogous sequences of the gamma or primary bacterial att sites. The core sequences of the left prophage att site is completely homologous to the wild-type core and also shows the same 8-base pair overlap with the thr operon attenuator. The position of the thr operon attenuator, relative to the left prophage att site, indicates that ribonucleic acid transcripts, initiated at a gamma promoter, are terminated efficiently at the thr attenuator. It is also possible that this prophage att site is able to undergo int dependent site-specific recombination which with another nearby secondary att site. Evidence is also presented which suggests that a base or sequence to the left of position -6 in the core is necessary fo excisive recombination.