A DELETION MUTANT IN THE HUMAN CYTOMEGALOVIRUS GENE ENCODING IE1(491AA) IS REPLICATION-DEFECTIVE DUE TO A FAILURE IN AUTOREGULATION

Human cytomegalovirus (CMV) replication begins with the expression of two regulatory proteins, IE1(491aa) and IE2(579aa), produced from diff erentially spliced transcripts under control of the ie1/ie2 promoter-e nhancer. A deletion mutation removing all 406 IE1(491aa)-specific amin o acids was engineered into the viral genome and this mutant (RC303 De lta Acc) was propagated on an IE1(491aa)-expressing human fibroblast c ell line (ihfie1.3). RC303 Delta Acc failed to replicate on normal hum an fibroblasts at low multiplicities of infection (mois). At mois >3 p laque-forming units per cell, virus replication and production of prog eny were comparable to wild type. However, at mois between 0.01 and 1, mutant virus replicated slowly on normal fibroblasts, a pattern that suggested initiation of productive infection required multiple hits. R eplication of RC303 Delta Acc correlated with the ability to express I E2(579aa), consistent with a role for IE1(491aa) in positive autoregul ation of the ie1/ie2 promoter-enhancer and with data suggesting that v irion transactivators compensate for the lack of IE1(491aa) under high moi conditions. ie1-deficient CMV should be completely avirulent, sug gesting its utility as a gene therapy vector for hematopoietic progeni tors that are normal sites of CMV latency.