The four classes of endogenous murine leukemia virus: structural relationships and potential for recombination.

The process by which leukemogenic viruses are generated during the lifetime of certain strains of mice is poorly understood. We have therefore set out to define all the murine leukemia virus-related endogenous proviruses of HRS/J mice. We have cloned 34 different proviral fragments and their flanking cellular sequences. These have been characterized by restriction enzyme analysis, by fingerprinting in vitro-synthesized RNA, and by DNA sequencing. We conclude that all the proviruses can be assigned into one of four different classes: the previously characterized ecotropic, xenotropic, and polytropic viruses, as well as a new class we have termed modified polytropic viruses. The xenotropic, polytropic, and modified polytropic classes are closely related to one another, but as a group they differ considerably from the ecotropic class. Sequence analyses show that both polytropic and modified polytropic sequences can contribute env sequences to recombinant viruses.