Detection of tetramerization domains in vivo by cooperative DNA binding to tandem lambda operator sites.

Chimeric proteins comprising the N-terminal DNA binding domain of lambda repressor fused to a fragment of a foreign protein have been used to detect oligomerization of the latter. Fusions containing dimeric and tetrameric leucine zipper domains can be distinguished based on their in vivo repressor activities on a pair of cat-lacZ reporter strains. Repressor fusions are unable to efficiently repress transcription from a synthetic promoter that overlaps a weak operator site; repression by tetrameric, but not dimeric, fusion proteins is increased by the presence of a strong, upstream operator site. To construct reporters we developed a shuttle system that allows rapid construction of single-copy operon fusions in E. coli, with both cat and lacZ as reporters.