A proposed mechanism for late reverse transcription in cells producing HIV-1 NC mutant particles. A- Newly made Gag and GagPol molecules (1) assemble using the genomic RNA and cellular membrane as platforms, (2) then wild type HIV-1 virions are produced by budding. These processes are facilitated by interactions between NC and cellular proteins (large black arrows). The core containing the genomic RNA is condensed with a cone-shaped structure. A limited level of viral DNA synthesis in producer cells and also anatural endogenous reverse transcription (NERT) can occur (see text). B- In cells producing HIV-1 Gag and GagPol with mutation in or deletion of the nucleocapsid CCHC zinc finger (1), assembly (2) and budding (grey arrow) are probably slowed down due to impaired interactions between NC and the viral RNA. They result in a partial delocalization of Gag in producer cells and a reduced level of newly made viral particles (grey arrow) (see text for references). The resulting virion core is formed of mature Gag proteins, but it is poorly condensed as seen by electron microscopy (see text for references). Taken together, these observations favor the notion that the NC CCHC mutations modify the kinetics of viral assembly which prevent core condensation and could explain, at least in part, why late-premature reverse transcription can readily take place in producer cells.