The HIV-1 nucleocapsid protein does not function as a transcriptional activator on its own cognate promoter

The human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) is a multifunctional, zinc finger-containing protein known to be involved in almost every step of the viral life cycle. We therefore examined the effects of NC in vivo as a transcription activator on the basal transcriptional activity of the HIV-1 U3 and Rous sarcoma virus (RSV) promoters, as well as HIV-1 long terminal repeats (LTRs) such as the U3R and U3RU5 regions, using promoter-fused reporter gene assays, Western blot analyses, and quantitative real time-polymerase chain reaction. From these studies, we found that the basal transcriptional levels of the HIV-1 U3 and RSV promoters were barely enhanced by the presence of NC. Placing the U3R region upstream of reporter genes greatly increased transcriptional activity compared to that of the U3 promoter alone, and such activity was further increased by Tat expression. However, neither transcription driven by U3R itself nor Tat-mediated transcriptional activation of the U3R was further increased by the addition of NC. Similar results were also observed with U3RU5 of the HIV-1 LTR region in the presence of either NC or Gag protein. Thus, these results indicate that the HIV NC protein is unable to act as a transcriptional activator on its cognate and possibly other retroviral promoters.