A snapshot of the λ T4rII exclusion (Rex) phenotype in Escherichia coli.

The lambda (λ) T4rII exclusion (Rex) phenotype is defined as the inability of T4rII to propagate in Escherichia coli lysogenized by bacteriophage λ. The Rex system requires the presence of two lambda immunity genes, rexA and rexB, to exclude T4 (rIIA-rIIB) from plating on a lawn of E. coli λ lysogens. The onset of the Rex phenotype by T4rII infection imparts a harsh cellular environment that prevents T4rII superinfection while killing the majority of the cell population. Since the discovery of this powerful exclusion system in 1955 by Seymour Benzer, few mechanistic models have been proposed to explain the process of Rex activation and the physiological manifestations associated with Rex onset. For the first time, key host proteins have recently been linked to Rex, including σE, σS, TolA, and other membrane proteins. Together with the known Rex system components, the RII proteins of bacteriophage T4 and the Rex proteins from bacteriophage λ, we are closer than ever to solving the mystery that has eluded investigators for over six decades. Here, we review the fundamental Rex components in light of this new knowledge.