All normal retroviral particles contain a dimer of genomic RNA. The physical structure of the dimer, and of the linkage between the two monomers, is not understood. We reported some years ago that the viral nucleocapsid protein alters the linkage between the monomers when it is released from Gag during viral maturation. This stabilization of the dimeric linkage results from the nucleic acid chaperone activity of nucleocapsid. We have continued to probe the structure of viral RNA within authentic virus particles, including a recent analysis of the dimer linkage in the immature RNA present in immature murine leukemia virus particles. _____Retroviral nucleocapsid proteins are highly active nucleic acid chaperones. The mechanism of this crucial activity is not well understood. We analyzed the binding of HIV-1 nucleocapsid to a very short (8-base) oligodeoxynucleotide in great detail, using several biophysical techniques. We found that a single nucleocapsid molecule can bind simultaneously to two nucleic acid molecules; conversely, a single nucleic acid molecule can bind two nucleocapsid molecules. It seems likely that the ability of the protein to interact with more than one nucleic acid molecule is a critical element in its nucleic acid chaperone activity. _____We are currently analyzing packaging signals in both HIV-1 and MLV viral RNAs, using both genetic and structural methods. _____Recent Accomplishments and Current Research: _____a. Genetic analysis of gammaretroviral 5' untranslated regions. In collaboration with Drs. Kevin Weeks and Robert Gorelick, we have defined the dimerization/packaging signal of MLV RNA in unprecedented detail. We will perform further mutagenic analysis and do comparative studies on other gammaretroviruses. We have also analyzed the structure of immature MLV dimeric RNA. We will also perform mutagenic studies on the packaging signal of HIV-1 RNA. _____b. Characterization of nucleic acid chaperone activity of MLV Gag protein. In collaboration with Dr. Karin Musier-Forsyth, we have found that the MA domain can interfere with the annealing activity of HIV-1 Gag. We will test whether this is also true of MLV Gag, despite the extended conformation of the latter. _____c. Analysis of selective packaging of cellular mRNAs. A few cellular mRNAs are greatly enriched in retrovirus particles. In collaboration with Drs. Sean Davis and J. Robert Hogg, we will try to identify the packaging signal in one of them, viz. ASB-1 mRNA, and to determine whether it is packaged in dimeric form. _____d. Solution studies of binding of HIV-1 Gag to nucleic acids. The binding of Gag proteins to nucleic acids is very poorly characterized to date. We are performing a detailed analysis of the binding of recombinant HIV-1 Gag protein to specific RNAs, using fluorescence correlation spectroscopy. _____Patents Linked to Project: U.S. Patent #5,674,720: Design and Construction of Noninfectious Human Retroviral Mutants Deficient in Genomic RNA; issued October 7, 1997; Robert J. Gorelick, Larry O. Arthur, Alan Rein, Louis E. Henderson, and Stephen Oroszlan. This patent describes mutants of HIV-1 that are structurally normal but noninfectious; these mutants could potentially be considered as vaccine constituents. _____[Corresponds to Rein Project 2 in the October 2011 site visit report of the HIV Drug Resistance Program (renamed HIV Dynamics and Replication Program in 2015)]