De-novo promoters emerge more readily from random DNA than from genomic DNA

Promoters are DNA sequences that help to initiate transcription. Point mutations can create de-novo promoters, which can consequently transcribe inactive genes or create novel transcripts. We know little about how de-novo promoters emerge in genomic DNA, especially compared to random DNA that has never been subjected to selection. Here, we assayed the promoter activity of 17,129 random, synthetic DNA sequences and 91,866 E. coli genomic DNA sequences. Genomic DNA encodes ~1.3 times more promoters than random DNA. We then studied 584,573 point mutations in 225 random and 60 genomic sequences, and asked how they cause the emergence of de-novo promoters. We find that de-novo promoters emerge ~3 times more readily from random DNA than from genomic DNA. The reason is that the genome contains fewer proto-binding sites for transcriptional activators than random DNA. Our work shows that the evolutionary history of a DNA sequence introduces substantial biases in its evolutionary potential, especially in the likelihood that mutations create new and potentially adaptive transcripts.