Figure 8. Two major functional networks controlling initiation by scanning.

In our genetic experiments, additive/suppressive effects are mainly observed between alleles predicted to function by alteration to initiation efficiency (rpb1 and tested alleles of TFIIB/TFIIF). Multiple lines between classes indicate allele-specific interactions between a factor and individuals of an allele class, for example, sub1∆ and ssl2 LOF alleles. In contrast to interactions within the ‘efficiency’ network, widespread epistasis was observed between ssl2 and other factors as predicted for interactions between processivity and efficiency alleles. sub1∆ generally shows additivity/suppression with ssl2 alleles, consistent with it functioning as a scanning processivity factor. Unique lethal interactions between ssl2 loss-of-function (LOF) upstream shifter F498L and sua7-1 and sub1∆ indicate distinct behavior within the ssl2 LOF class.