Efficient transcription of the EBER2 gene depends on the structural integrity of the RNA

A 3'-truncated EBER2 RNA gene, although containing all previously identified promoter elements, revealed drastically reduced transcription rates in vitro and in vivo when fused to a heterologous terminator sequence. Inactivations were also observed with double point mutations affecting 5'- or 3'-end sequences of the EBER2 gene. However, wild-type activity of these mutants could be restored by compensatory mutations of the opposite strand of the EBER2 RNA sequence. A similar rescue was achieved with the 3'-truncated EBER2 gene, if the heterologous terminator was adapted for complementarity to the initiator element of the construct. Yet, double-strandedness alone of the RNA ends was not sufficient for high transcriptional activity of these gene constructs. Rather, the use of a nonrefoldable spacer, separating the 5'- and 3'-stem-loop structures, demonstrated that spatial proximity of the ends of EBER2 RNA was required. Furthermore, decay kinetics of wild-type and mutant RNA synthesized in vitro indicated that the effects observed could not be explained by altered transcript stability. Finally, single-round transcription confirmed that the reduced expression of mutant genes was not caused by decreased primary initiation reactions. In addition, differential sarcosyl concentrations demonstrated that the rate of reinitiation clearly was affected with the mutant EBER2 genes. Together, these results indicate that the secondary structure of this viral RNA represents a major determinant for efficient transcription of the EBER2 gene by host cell RNA polymerase III.