The genome of the human immunodeficiency virus type 1 (HIV-1) is highly heterogeneous. Some of this genomic variability most likely translates into the biologic and serologic differences observed among various strains of HIV-1. HIV-1 isolates vary in their ability to infect different cell types ( e.g. established T lymphoid cell lines, primary macrophages, human fibroblast cells), to replicate to high titers, and to cause damage to the infected cell. Difference in the isolates' susceptibility to serum neutralization have also been observed. Some of these in vitro differential properties have been correlated with pathogenicity of the virus in vivo. Defining the functional gene(s) of HIV-1 that control these biologic and serologic properties could help in the understanding of HIV-1 pathogenesis and in designing strategies for anti-viral therapies. This proposal is aimed at identifying the genes and domains within genes that determine the host range, cytopathogenicity and sensitivity to serum neutralization of HIV-1. Results obtained from our preliminary studies with recombinant viruses generated between different biologically active HIV-1 clones indicate that viral entry, cytopathology and sensitivity to neutralization are determined by the envelope gene, whereas kinetics of replication appear to be controlled by the LTR region. We propose to generate additional recombinant viruses between parental clones already used in our preliminary studies in order to define functional domains within the HIV-1 envelope gene and LTR that affect these biologic and serologic properties (Specific aims #1). We will also molecularly clone and sequence other HIV-1 isolates of interest (Specific aim #2) for the construction of new recombinant viruses (Specific #3). These will be useful in further verification of those genes and domains defined in Specific aim #1, and in the potential identification of additional regions of functional significance. The effects of specific changes within the functional domains will be investigated by site-directed mutagenesis (Specific #4). Finally, in vitro transient gene expression assays (Specific aim #5) will be used to explore further the role of the viral LTR and tat gene in determining the host range of HIV-1.