Four dimers of lambda repressor bound to two suitably spaced pairs of lambda operators form octamers and DNA loops over large distances

Transcription factors that are bound specifically to DNA often interact wit h each other over thousands of base palm [1,2]. Large DNA loops resulting f rom such interactions have been observed in Escherichia coil with the trans cription factors deoR [3] and NtrC [4], but such interactions are not, as y et, well understood. We propose that unique protein complexes, that are not present in solution, may form specifically on DNA, Their uniqueness would make it possible for them to interact tightly and specifically with each ot her, We used the repressor and operators of coliphage lambda to construct a model system in which to test our proposition. lambda repressor is a dimer at physiological concentrations, but forms tetramers and octamers at a hun dredfold higher concentration. We predict that two lambda repressor dimers form a tetramer in vitro when bound to two lambda operators spaced 24 bp ap art and that two such tetramers interact to form an octamer, We examined, i n vitro, relaxed circular plasmid DNA in which such operator pairs were sep arated by 2,850 bp and 2,470 bp. Of these molecules, 29% formed loops as se en by electron microscopy (EM), The loop increased the tightness of binding of lambda repressor to lambda operator, Consequently, repression of the la mbda PR promoter in vivo was increased fourfold by the presence of a second pair of lambda operators, separated by a distance of 3,600 bp.