Table 1.

Predicted density of low-occupancy, nonfunctional DNA binding by transcription factors in vivo

SpeciesDNA length per cell (Mb)*Accessible genome (Mb)Transcription factorMolecules per cellNonfunctional molecules per kb accessible genome
E. coli4.64.6 (38)Lac I10 (10)0.002 (13, 15)
E. coli4.64.6 (38)Median300 (39)0.065
D. melanogaster360.024.0 (20)Median50,000 (3)2.1
H. sapiens6,400.064.0 (40)Median120,000 (3)1.9
  • *E. coli DNA content is for a haploid cell, D. melanogaster and Homo sapiens is for diploid cells.

  • In E. coli cells, at any instant 20% of Lac I transcription factor molecules make functional contacts with target cis-regulatory DNA recognition sites, 10% are not bound to DNA, and 70% make low-occupancy, nonfunctional DNA interactions (13, 15). Assuming that the same ratios apply for other transcription factors, the predicted density of low-occupancy interactions at accessible portions of the genome are given for a transcription factor expressed at the median concentration typical for a transcription factor in E. coli, D. melanogaster, and H. sapiens.