1TLF | A:162-290,A:321-336; B:162-290,B:321-336; C:162-290,C:321-336; D:162-290,D:321-336; A:61-161,A:291-320; B:61-161,B:291-320; C:61-161,C:291-320; D:61-161,D:291-320 | UNPRECEDENTED QUATERNARY STRUCTURE OF E. COLI LAC REPRESSOR CORE TETRAMER: IMPLICATIONS FOR DNA LOOPING |