Post-transcriptional regulation of miR-27 in murine cytomegalovirus infection.

In mammals, microRNAs (miRNAs) can play diverse roles in viral infection through their capacity to regulate both host and viral genes. Recent reports have demonstrated that specific miRNAs change in expression level upon infection and can impact viral production and infectivity. It is clear that miRNAs are an integral component of viral-host interactions, and it is likely that both host and virus contain mechanisms to regulate miRNA expression and/or activity. To date, little is known about the mechanisms by which miRNAs are regulated in viral infection. Here we report the rapid down-regulation of miR-27a in multiple mouse cell lines as well as primary macrophages upon infection with the murine cytomegalovirus. Down-regulation of miR-27a occurs independently from two other miRNAs, miR-23a and miR-24, located within the same genomic cluster, and analysis of pri-miRNA levels suggest that regulation occurs post-transcriptionally. miR-27b, a close homolog of miR-27a (20/21 nucleotide identity), also decreases upon infection, and we demonstrate that both miR-27a and miR-27b exert an antiviral function upon over-expression. Drug sensitivity experiments suggest that virus entry is not sufficient to induce the down-regulation of miR-27 and that the mechanism requires synthesis of RNA. Altogether, our findings indicate that miR-27a and miR-27b have antiviral activity against MCMV, and that either the virus or the host encodes molecule(s) for regulating miR-27 accumulation, most likely by inducing the rapid decay of the mature species.

Discrete clusters of virus-encoded micrornas are associated with complementary strands of the genome and the 7.2-kilobase stable intron in murine cytomegalovirus.

The prevalence and importance of microRNAs (miRNAs) in viral infection are increasingly relevant. Eleven miRNAs were previously identified in human cytomegalovirus (HCMV); however, miRNA content in murine CMV (MCMV), which serves as an important in vivo model for CMV infection, has not previously been examined. We have cloned and characterized 17 novel miRNAs that originate from at least 12 precursor miRNAs in MCMV and are not homologous to HCMV miRNAs. In parallel, we applied a computational analysis, using a support vector machine approach, to identify potential precursor miRNAs in MCMV. Four of the top 10 predicted precursor sequences were cloned in this study, and the combination of computational and cloning analysis demonstrates that MCMV has the capacity to encode miRNAs clustered throughout the genome. On the basis of drug sensitivity experiments for resolving the kinetic class of expression, we show that the MCMV miRNAs are both early and late gene products. Notably, the MCMV miRNAs occur on complementary strands of the genome in specific regions, a feature which has not previously been observed for viral miRNAs. One cluster of miRNAs occurs in close proximity to the 5' splice site of the previously identified 7.2-kb stable intron, implying a variety of potential regulatory mechanisms for MCMV miRNAs.