Purine-rich coding sequences guard bacterial genes from Rho termination during runaway transcription

Using a genome-wide Rho-termination screen in Bacillus subtilis, the study shows that coding strands are purine-rich to shield runaway transcription from Rho-dependent termination, while antisense regions are pyrimidine-rich, making them targets for Rho. In rho-containing Bacilli this purine bias shapes codon usage and constrains foreign gene expression; rho-less species relax the bias and can tolerate more pyrimidines. Recoding a gene to be more purine-rich suppresses Rho termination, and foreign sequences such as HGH lose expression in B. subtilis unless recoded. The findings reveal a sequence-based constraint on genome evolution beyond replication biases.