RESUMO
DNA can act as a scaffold for the cooperative binding of protein oligomers. For example, the phage 186 CI repressor forms a wheel of seven dimers wrapped in DNA with specific binding sites, while phage λ CI repressor dimers bind to two well-separated sets of operators, forming a DNA loop. Atomic force microscopy was used to measure transcription elongation by Escherichia coli RNA polymerase (RNAP) through these protein complexes. 186 CI, or λ CI, bound along unlooped DNA negligibly interfered with transcription by RNAP. Wrapped and looped topologies induced by these scaffolded, cooperatively bound repressor oligomers did not form significantly better roadblocks to transcription. Thus, despite binding with high affinity, these repressors are not effective roadblocks to transcription.
