Domains of the lac Repressor

There are four identical subunits that form nine domains. The polypeptide chain starts in the red domain (or yellow) and travels back and forth three times to make the blue domain (or green). Finally, one helix at the C-terminus from each subunit forms a four-helix bundle, the ninth domain.

Friedman AM, Fischmann TO, Steitz TA.
Crystal structure of lac repressor core tetramer and its implications for DNA looping.
Science. 1995 Jun 23;268(5218):1721-7. Pubmed

1tlf PDB file