The expression of a viral microRNA is regulated by clustering to allow optimal B cell transformation

The Epstein-Barr virus (EBV) transforms B cells by expressing latent proteins and the BHRF1 microRNA cluster. MiR-BHRF1-3, its most transforming member, belongs to the recently identified group of weakly expressed microRNAs. We show here that miR-BHRF1-3 displays an unusually low propensity to form a stem-loop structure, an effect potentiated by miR-BHRF1-3's proximity to the BHRF1 polyA site. Cloning miR-BHRF1-2 or a cellular microRNA, but not a ribozyme, 5' of miR-BHRF1-3 markedly enhanced its expression. However, a virus carrying mutated miR-BHRF1-2 seed regions expressed miR-BHRF1-3 at normal levels and was fully transforming. Therefore, miR-BHRF1-2's role during transformation is independent of its seed regions, revealing a new microRNA function. Increasing the distance between miR-BHRF1-2 and miR-BHRF1-3 in EBV enhanced miR-BHRF1-3's expression but decreased its transforming potential. Thus, the expression of some microRNAs must be restricted to a narrow range, as achieved by placing miR-BHRF1-3 under the control of miR-BHRF1-2.