Extracytoplasmic (ECF) ? factors, the largest class of alternative ? factors, are related to primary ? factors, but have simpler structures, comprising only two of six conserved functional modules in primary ? factors: region 2 (?R2) and region 4 (?R4). Here, we report crystal structures of transcription initiation complexes containing Mycobacterium tuberculosis RNA polymerase (RNAP), <
em>
M. tuberculosis<
/em>
ECF ? factor ?L, and promoter DNA. The structures show that ?R2 and ?R4 of the ECF ? factor occupy the same sites on RNAP as in primary ? factors, show that the connector between ?R2 and ?R4 of the ECF ? factor?although shorter and unrelated in sequence?follows the same path through RNAP as in primary ? factors, and show that the ECF ? factor uses the same strategy to bind and unwind promoter DNA as primary ? factors. The results define protein-protein and protein-DNA interactions involved in ECF-?-factor-dependent transcription initiation.