Host-parasite interactions: recent developments in the genetics of abortive phage infections

Recent progress towards the elucidation of the mechanisms of phage exclusion systems that lead to abortive infections is described. Abortive infections constitute model systems for the study of host-parasite interactions, but until the 1980s such studies were confined largely to descriptions of a plethora of physiological dysfunctions. The use of cloning vectors containing the genes involved in the exclusion phenomena have helped to reduce the complexity of the abortive infection while maintaining many of its natural biological characteristics. The use of these vectors has also led to the development of a convenient assay for the identification of the targets of an exclusion system. Plasmids expressing a target gene are frequently found to be unable to transform a cell that contains the exclusion system. This article focuses on two classic abortive infections, the exclusion of T4 rII mutants by lambda rex+ lysogens and of wild-type T7 by cells containing the plasmid F. In addition, some less well understood phage exclusion systems are described, since they share a number of features with the T4 and T7 systems. Elucidation of the normal functions of the components involved in some abortive infections is also helping our understanding of the underlying mechanisms, and it seems likely that one or more of these model systems will soon be fully described.