Coexistence of nucleosomal and various non-nucleosomal chromatin configurations in cells infected with herpes simplex virus

Coexistence of four different forms of chromatin was observed by electron microscopy in nuclear spread preparations of monkey kidney cells during late stages of infection with herpes simplex virus (HSV-1 AMG). Besides typical nucleosomal (i) chromatin, thin (3-5 nm) strands morphologically indistinguishable from protein-free DNA were frequent, without (ii) or with (iii) sparse 10-22 nm large granules different from nucleosomes. In addition, uniformly thick (mean 17 nm), heavily stained chromatin strands (iv) were seen. The non-nucleosomal character of types (iii) and (iv) chromatin was also demonstrated by their resistance to histone removal in Sarkosyl and heparin. All four forms were seen in capsid-associated HSV-DNA molecules, and various combinations of these forms occurred in adjacent regions of the same DNA molecule, including the vicinity of replication branch points. Especially frequent were regions of chromatin types (ii) or (iii) alternating with thickly coated intercepts of type (iv) chromatin, the latter often displaying "bubble"-like strand separations. The appearance of chromatin types (ii)-(iv) was dependent on viral replication. These chromatin arrays were compared with structures observed in purified HSV-DNA from these cells. Patterns of single-stranded regions were found in HSV-DNA that were similar to those observed in the thickly coated type (iv) chromatin. It is concluded that, in these nuclei, non-nucleosomal arrangements can be formed, at least on viral DNA, under conditions of continued DNA synthesis and inhibited protein synthesis, and that single-stranded DNA is packed into a characteristic thick strand of non-nucleosomal chromatin by association with a special, probably virus-coded protein.