We have suggested previously that RNA secondary structure of body TRS regions contributes to their attenuating potential and thereby determines the relative portion of the nascent minus strands that is transferred to the leader TRS in the template (Pasternak et al., 2000) . At present, it is unknown whether EAV body TRSs are part of an RNA structural motif that is essential for body TRS function, or whether they are recognized by a protein factor in a sequence-speci®c manner. However, the latter seems less likely than the former, since even LB4 (Figure 2 ), in which ®ve TRS nucleotides were substituted, still produced some sg RNA7, although~30-fold less than the wild-type control. The fact that some sequences in the EAV genome match the leader TRS perfectly, but are not used for sg mRNA synthesis, also argues against the recognition of a speci®c sequence (Pasternak et al., 2000) . More probably, mutagenesis of the RNA7 body TRS disturbed an RNA structure that is necessary for its function. This could, for example, explain the fact that the BU 6 C substitution reduced the amount of RNA7 by 20-fold (and could not be rescued by the same mutation in the leader TRS), whereas the wild-type RNA6 body TRS contains a C at the same position. If a protein factor were involved in sequence-speci®c TRS recognition, then one would expect it to recognize all TRSs similarly. If RNA structure is important for recognition by such a protein, then the BU 6 C substitution probably disturbs a structural motif of the RNA7 TRS, which is not present in the RNA6 TRS. On the other hand, conservation of part of the TRS in other arteriviruses suggests a sequence-speci®c recognition. Further studies are required to distinguish between these possibilities.