Human cytomegalovirus UL21.5 gene is expressed as an "early late" gene in cultured human fibroblasts

The human cytomegalovirus (CMV) UL21.5 gene encodes a secreted glycoprotein of unknown function. Both the UL21.5 protein and mRNA accumulate in abunda nce at late stages of infection making the RNA an attractive target for dia gnosis of active CMV infection. The UL21.5 was originally described as a 's pliced late' gene (SLG) (Rawlinson and Barrell, J. Virol. 67, 5502 (1993)). However, we found that the the UL21.5 mRNA was detectable in CMV infected patients before the onset of CMV DNA replication (Boriskin et al., J. Clin. Virol., in press). Here, we re-examined the UL21.5 mRNA kinetic class in C MV infected human fibroblast culture using a RNAse protection assay and RT PCR. The UL21.5 mRNA was detectable before the "true late" UL75 RNA, was re sistant to a CMV DNA replication inhibitor but moderately sensitive to inhi bitors of protein synthesis. In the presence of protein synthesis inhibitor s the UL21.5 mRNA was detectable only by a nested reverse transcription - P CR (RT-PCR) with the bulk of it in unspliced form. This suggests that splic ing factors for UL21.5 mRNA are encoded by the virus rather than by the cel l. Our results indicate that UL21.5 should be defined as an "early-late" ra ther than a "late" (L) CMV gene.