In the process of generating restriction endonuclease site deletion mutants, we have isolated and characterized three mutants of herpes simplex virus type 2 (HSV-2), strain HG52, with large genomic deletions in U and TR. The deleted sequences (7·5 kb) extend from 0·94 map coordinates (m.c.) to 0·99 m.c. and are presumed to contain the HSV-2 gene equivalents of US10, 11 and 12, one copy of immediate early (IE) gene 3 and one copy of an origin of replication (ORI). One of the mutants (HG52X163X12) has a simple deletion whereas in the two others (HG52X163X14 and HG52X163X21) the deleted sequences have been replaced by inverted duplications of U/IR sequences between 0·83 and 0·91 m.c. such that the molecules have short region inverted repeats extended by 6 kb on either side. All three are viable, stable and grow in tissue culture indicating that the polypeptides coded by the HSV-2 genes equivalent to US10, 11 and 12 are non-essential for lytic growth in BHK21/C13 cells. In addition the lack of one copy of the HSV-2 equivalent of IE gene 3 and ORI in HG52X163X12 shows that only one copy of each suffices for viability. However the temperature restriction of the mutants at 38·5 °C suggests that one or more of the polypeptides coded by the deleted sequences may be required in conjunction with another polypeptide(s) for viral growth or stability at the higher temperature.


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