Size, number, and distribution of poly G binding sites on the separated DNA strands of coliphage T7

Abstract

Poly G binds to only one of the two complementary strands of coliphage T7 DNA, as detected by measuring the buoyant density increments in the CsCl gradient and by trapping of the [ 3H]poly G · denatured DNA complexes on nitrocellulose membranes. The poly G binding sites on DNA most probably consist of deoxycyti-dylate-rich clusters, 15–40 nucleotides long, as judged from the “melting” temperatures of the complexes and the sedimentation rates of the [ 3H]poly G fragments recovered from the ribonuclease T 1-treated complexes. Since the ribonuclease T 1-resistant fraction of the poly G represents 3.3% of the complex, one could estimate that there are approx. 30–75 poly G binding sites per one T7 DNA molecule. These sites appear to be evenly distributed along the T7 DNA, as shown by the patterns of interaction between poly G and fragmented, denatured DNA. As reported earlier by Summers and Szybalski 3, only the poly G binding (“heavy”) DNA strand is transcribed into RNA during all phases of T7 phage development.