476-480 , January 1989 Biochemistry In vivo DNA loops in araCBAD : Size limits and helical repeat ( supercoiling/twist/periodicity / linking number deficit/looping energetics ) DONG-HEE LEE AND ROBERT F. SCHLEIF Biochemistry Department , Brandeis University , Waltham , MA 02254 Communicated by Alexander Rich , October 14 , 1988 ABSTRACT Formation of a DNA loop by AraC proteins bound at the aral and araO2 sites , whose center-to-center distance is 211 base pairs , is necessary for repression of the araBAD promoter , PBAD , of Escherichia coli .