Properties and directionality of intragenic promoters in E. coli

The histone-like nucleoid structuring (H-NS) protein targets and binds AT-rich DNA. Oligomerisation of H-NS along AT-rich genes represses transcription from spurious intragenic promoters within the coding sequences of AT-rich genes. In this work, we sought to better understand why promoters occur so frequently within AT-rich DNA. To do this, we compared the properties of (i) canonical promoters (ii) promoters within H-NS bound genes and (iii) promoters generated by random combinations of nucleotides. We have identified and analysed many active intragenic promoters distributed within AT-rich genes in the E. coli genome. We show that these promoters, and promoters from randomly generated AT-rich DNA, differ from canonical promoters in several ways. In particular, spurious promoters are often dependent on AT-tracts upstream of the promoter −10 element. These AT-tracts play a key role by altering DNA curvature and facilitating interactions between the promoter and RNA polymerase sigma factor. Promoter regions inside genes are also often bidirectional.