Isolation of a short, cytosine-rich repeating unit from the DNA of Escherichia coli

Hybridization of heterologous nucleic acids has provided the means for isolating a repeating sequence which is located next to template regions of DNA. Separated single strands of 32P-labelled DNA from Escherichia coli were to a limited extent able to anneal with DNA of Micrococcus lysodeikticus immobilized on nitrocellulose membrane filters. The resulting hybrid was resistant to enzymes specific for unpaired strands, nuclease S1 (Aspergillus oryzae) and exonuclease I (E. coli). The E. coli DNA so hybridized was isolated and characterized. It contained all four bases with cytosine predominating; strand length was about 50-60 nucleotides. Since these units occupied about 1-2% of the length of the E. coli chromosome, they would have to be repeated about 2000 times in a single cell. Formation of the unusual hybrid was not diminished by prior saturation of the E. coli DNA with homologous 3H-labelled RNA. In fact both RNA and additional increments of DNA were detected on the filters approximately in a 1:1 ratio, showing that some of the repeating sequences were physically continuous with transcribed regions of DNA.