SV40 Chromatin Structure and Virus Assembly

The process of virion assembly plays an active role in modulating the structure of SV40 chromatin. Experiments with temperature-sensitive mutants of the major capsid protein (VP1) have shown that minichromosomes which contain an exposed regulatory region accumulate when the initiation of SV40 assembly is blocked. VP1, directly or through specific interactions with other proteins, alters the average spacing between nucleosomes and may be involved in generating a protected regulatory region in SV40 chromatin. The ENA sequences near or between positions 640 and 875 interact with capsid proteins in vivo and appear to contain signals used for the initiation of shell assembly. A ENA topoisomerase is encapsidated during virion assembly. This enzyme is probably responsible for the difference in SV40 ENA linking number, observed previously, between unencapsidated chromatin and virion derived chromatin.