REP sequences: Mediators of the environmental stress response?

Repetitive Extragenic Palindromic (REP) sequences are highly conserved, structured, 35- to 40-nt elements located at ∼500 positions around the Escherichia coli chromosome. They are found in intergenic regions and are transcribed together with their upstream genes. Although their stable stem-loop structures protect messages against exoribonuclease digestion, their primary function has remained unknown. Recently, we found that about half of all REP sequences have the potential to stall ribosomes immediately upstream of the termination codon, leading to endonucleolytic cleavage of the mRNA, and induction of the trans-translation process. As a consequence, the mRNA and almost completed protein are degraded, and protein production from the affected gene is down-regulated. The process is critically dependent on the location of the REP element, with an effect only if it is within 15 nt of the termination codon. Using nrdAB as a model, we found that its down-regulation is affected by RNA helicases. Elimination of 6 helicases lowered NrdA production further, whereas overexpression of any RNA helicase partially reversed the downregulation. UV stress completely reversed down-regulation of NrdA production. Analysis of genes containing a REP sequence within 15 nt of the termination codon revealed that most, if not all, are up-regulated by environmental stress, as are RNA helicases. Based on these findings, we propose that REP-dependent downregulation serves as a mechanism to allow a rapid response to environmental stresses whereby RNA helicases partially open the REP elements enabling ribosomes to complete translation immediately increasing protein production from the affected genes.