CHARACTERIZATION OF A MOLECULAR CLONE OF A HIGHLY INFECTIOUS AVIAN-LEUKOSIS VIRUS

A highly infectious avian leukosis virus (ALV) has been molecularly cl oned in a Lambda phage and sequenced. In order to manipulate the genom e of this ALV and characterize the genetic determinants responsible fo r the high infectivity phenotype, a recombinant plasmid DNA with the t wo LTR provirus was constructed. Upon transfection of avian cells with this ALV DNA construct infectious viruses were produced as soon as 4 h after transfection and virus titer was 10(5) iu/ml after 24 h while that of the extensively characterized Rous sarcoma virus (RSV) was onl y 10(1) iu/ml. Nucleotide sequence comparison of the ALV genome with t hat of RSV indicates that the major differences are in the Gag gene wh ile the Env gene is identical to that of RSV subgroup A. To map the ma jor genetic determinants responsible for the high infectivity phenotyp e of this ALV, ALV/RSV chimeric viruses were constructed and their phe notype investigated. Data indicate that the high infectivity of this A LV is mainly associated with Gag and this could be due to a rapid proc essing of the Gag polyprotein precursor during virion assembly. Sequen ce analyses further suggest that this ALV isolate was generated by rec ombination between endogenous and exogenous viruses, and a possible me chanism of this recombination is discussed. This ALV molecular clone i s presently used to develop improved helper cells and retroviral vecto rs for gene transfer in avian cells.