Proximity of deoxyadenylic-thymidylic acid clusters to origin of DNA replication in E. coli 15T⁻ cells

Abstract

CLUSTERS of deoxyadenylic-thymidylic (dA-T) and deoxy-cytidylic-deoxyguanylic acid (dC-dG) residues have been detected in DNAs from many sources^1,2. The purine or pyrimidine clusters in denatured DNA can hybridise with complementary polyribonucleotides and the greater density of the complexes means that they can be separated from unreacted DNA by isopycnic centrifugation in CsCl or Cs₂SO₄ (refs 1 and 2). These clusters are estimated to be 10–100 residues long and are distributed throughout DNA at intervals of 10⁵–10⁶ daltons (ref. 3). The dC-dG clusters have been implicated in the regulation of transcription in viruses^4–8 bacteria^1,9 and in nuclei (P. K. Bhattacharya and H.K. unpublished) from eukaryotic cells (for reviews see refs 3 and 10). Less is known about the function of clusters of dA–T residues which are as common in most DNAs¹. Although in adenoviruses the presence of dA-T clusters has been correlated with the oncogenic potential¹¹ and Borst and Ruttenberg¹² found runs of dT on the transcribed strand of mitochondrial DNA no further efforts have been made to associate regulatory or other functions with the presence of these clusters. We now report observations which suggest that clusters of dA-T may be involved in the initiation of DNA synthesis. The rationale for our investigation was that if such clusters were detected at or near the replication origin it would be consistent with the proposition that they function in the initiation of DNA replication.