The pathway of E. coli RNA polymerase-promoter complex formation was probed by characterization of low temperature intermediates at the major coat protein promoter of phage fd DMA. Three different complexes could be distinguished. One of them represents the active 'open' complex, the other two have to be regarded as 'closed'. The promoter contacts of the lower temperature complexes were totally comprised within the contacts of the higher temperature complexes. Increase in temperature led to extension of contacts into the downstream direction, while the upstream border of the complexes remained virtually unchanged. Only in the 'open' complex contacts were extended beyond the start site of transcription.