ACTIVE-CENTER REARRANGEMENT IN RNA-POLYMERASE INITIATION COMPLEX

His1237 in the beta subunit of Escherichia coli RNA polymmerase marks the ''5' face'' of the active center since it can be cross-linked to t he gamma-phosphate of the priming substrate. It is demonstrated that R NA chains up to 9 nucleotides in length can be synthesized using His12 37-cross-linked nucleotide as a primer. Thus, a substantial mass of RN A can be accommodated in the active center between His1237 and the sit e of catalysis that remains juxtaposed to the growing 3' end. The appa rent ''filling'' of the active center with RNA precedes promoter clear ance and suggests a mechanism of coupling between catalysis and saltat ory translocation of RNA polymerase.