Bacteriophage λ and its Genetic Neighborhood

Abstract

Abstract A P4 virion is made of a linear, double-stranded, cohesiveended DNA molecule 11.6 kb long that is encapsidated into a tailed, icosahedral protein head (figure 26-1). The genome organization of P4 is diagrammed in figure 26-3A and the known genes and sites are detailed in table 26-1. Upon infection of the E. coli host, P4 DNA circularizes through its cohesive ends. The genetic vegetative map of P4 is circular (8, 27) and reflects the physical structure of the replicating DNA molecule, whereas the integrated prophage genome is a circular permutation of the mature DNA (17a). All the functions required for lysogenic, lytic, and plasmid development are located in the right 80% of the genome and include the origin of replication, ori1 (63), the two main a and sid operons which are transcribed divergently from the ori1 site (26, 30, 50), the prophage integration att site (17a, 78), and the int (integrase) gene in a monocistronic operon located to the left of att.As in many episomal elements, a ‘‘nonessential region’’ that carries functions not related to the P4 life cycle (gop, b, cII; table 26-1) is found adjacent to the att^int integration module (17a, 41). These genes, organized in the two constitutively expressed operons to the left of att^int, may be deleted without affecting lysogenic, lytic, or plasmid development (41, 58).