A nucleosome-free region in SV40 minichromosomes

Simian virus 40 (SV40) DNA is found in infected cells in the form of a minichromosome 1 . It possesses a beaded structure composed of cellular histones and supercoiled viral DNA in a molecular complex which is very similar to that of cellular chromatin 2–4 . The similarity between these structures, and the fact that the major viral functions take place in cell nuclei via cellular machinery, have made SV40 an attractive model system in which to study the organization and expression of the more complex eukaryotic chromatin. Early studies on SV40 chromatin, attempting to determine the precise distribution of the histone beads (nucleosomes) along the DNA, indicated that the nucleosomes were randomly distributed relative to the viral DNA sequences 5–8 . This picture has recently been altered by a study indicating that a region close to the viral origin of replication is particularly sensitive to nuclease digestion 9–13 . Using a variety of nucleases, 10–13 , the nuclease-sensitive region has been shown to map on a stretch of the genome beginning at the viral origin of replication and continuing about 400 base pairs into the ‘late’ side of the DNA. The simplest interpretations of the presence of a nuclease-sensitive region are that this region is either deficient in nucleosomal protein or that it contains a peculiar arrangement of these protein structures. We have analysed SV40 minichromosomes in the electron microscope in an attempt to determine whether alterations in the gross nucleoprotein structure of this region could be visualized. We report here that such an alteration does exist and that about 25% of SV40 minichromosomes observed contained a region of DNA between 0.67 and 0.75 map units which is not organized into the typical nucleosome beaded structure.