Virus host interactions as studied in HIV-1 and SV40 model systems

Abstract

An understanding of the nuclear processes taking place in eukaryotic cells has been achieved using HIV-1 and SV40 viruses. Their simple genomes provide excellent model systems. HIV-1, a retrovirus has been used to study the interaction of regulatory proteins with the negative regulatory element (NRE) within the long terminal repeat (LTR) of HIV-1. Three cDNA clones have been isolated by screening a HeLa expression library. One of the clones encodes NF-IL6, a transcription factor previously identified as an activator of Interleukin-6 (IL-6) gene expression in response to Interleukin-1 (IL-I) induction. NF-IL6 binds to multiple sites within the HIV-1 LTR and may play an important role in activating HIV-1 expression in response to IL-1 and IL-6 signal transduction. The second clone has been analyzed and examination of its amino acid sequence strongly suggests that it represents a DNA binding protein. SV40 has been used as a model system to study eukaryotic chromatin structure, since the viral genome forms a minichromosome similar to that of higher organisms. Location of nucleosomes assembled in vivo on the strain wt776 has been determined using micrococcal nuclease digestion of SV40 chromatin combined with M13 cloning and DNA sequencing. Several nucleosomes have been identified at and near the replication origin, the enhancer sequence, the early and late transcription initiation sites, the termination region and regions containing bent DNA. A complex pattern of overlapping nucleosomal clones has been observed. The results indicate that the nucleosomes do not occupy unique positions. However, the distribution of nucleosomes does not appear to be random. A stable nucleoprotein complex, obtained during the isolation of SV40 virions has been analyzed and considered to be an intermediate of virus assembly. The proteins topoisomerase I, topoisomerase II and T antigen along with SV40 chromatin have been identified as components of the complex. Topoisomerase II and T antigen have been identified previously as components of the nuclear scaffold and hence the results suggest that SV40 chromatin is associated with the nuclear scaffold during DNA replication and virus assembly