attL

attL stands for left attachment site and refers to one of the two junctions formed when a bacteriophage genome integrates into a bacterial chromosome. In the lambda phage system, integration occurs via site-specific recombination between the phage attachment site attP and the bacterial attachment site attB, a reaction catalyzed by the phage-encoded integrase (Int) and aided by host factors. The recombination yields two new junctions, attL on the left and attR on the right, flanking the integrated prophage.

AttL is a hybrid DNA sequence that combines elements derived from attP and attB. Its exact sequence and length are system-specific, but its defining feature is that it marks the left boundary between phage and host DNA in the integrated state. The attL site, together with attR, delineates the ends of the prophage region within the bacterial chromosome.

Excision, the reverse process, involves recombination between attL and attR to restore the original attP and attB sites and to excise the prophage. Excision requires the phage-encoded integrase and the excisionase (Xis) protein, and may depend on additional host factors. The directionality of the reaction is governed by these proteins and the local DNA architecture.

In research and biotechnology, attL (along with attR, attP, and attB) is used to study site-specific recombination and to engineer reversible integration systems. These concepts underpin certain genetic tools and vectors that enable targeted DNA integration and controlled excision in microbial hosts.