Differentiating between effects at the level of primary TRS sequence and the level of leader±body duplex formation For some TRS nucleotides (2, 5 and 6, except in the case of DU 6 C), the RNA7 level of double mutants was clearly higher than that of the corresponding single mutants. This means that base pairing between these leader and body TRS nucleotides is involved in sg RNA synthesis. However, none of these double mutants reached the wild-type sg RNA7 level. In the other double mutants (all position 1, 3 and 4 mutants, and DU 6 C), in clear contradiction to the predictions of the`base pairing model', RNA7 synthesis was not signi®cantly restored. Moreover, a comparison of the values for the B and D mutants in Figure 3 showed that, for almost all of these mutants (e.g. the position 1 mutants), the amount of sg RNA7 produced by the double mutant appeared to be limited by the level allowed by the body TRS mutation. Sometimes the RNA7 level of the double mutant was even less than that of the leader mutant (DU 1 C, DA 3 G, DA 4 G or DU 6 C). Clearly, for these substitutions, restoration of the possibilities for leader±body duplex formation did not restore sg RNA synthesis. Apparently this is because the effect of body TRS mutations at the level of primary sequence or secondary structure can be`dominant' over the duplex-restoring effects of the double mutations.