Shutdown decay of mRNA

Although plasmid-borne and chromosomal toxin-antitoxin (TA) operons have been known for some time, the recent identification of mRNA as the target of at least two different classes of toxins has led to a dramatic renewal of interest in these systems as mediators of stress responses. Members of the MazF/PemK family, the so-called mRNA interferases, are ribonucleases that inhibit translation by destroying cellular mRNAs under stress conditions, while the founder member of the RelE family promotes cleavage of mRNAs through the ribosome. Detailed structures of these enzymes, often in complex with their inhibitors, have provided vital clues to their mechanisms of action. The primary role and regulation of these systems has been the subject of some controversy. One model suggests they play a beneficial role by wiping the slate clean and preventing wasteful energy consumption by the translational apparatus during adaptation to stress conditions, while another favours the idea that their main function is programmed cell death. The two models might not be mutually exclusive if a side-effect of prolonged exposure to toxic RNase activity without de novo synthesis of the inhibitor were a state of dormancy for which we do not yet understand the key to recovery. In this review, I discuss the recent developments in the rapidly expanding field of what I refer to as bacterial shutdown decay.