Summary mRNA degradation affects virtually all cellular activities by limiting the number of times each mRNA can be translated into protein molecules. By affecting protein synthesis, mRNA degradation allows organisms to adapt to changing environmental conditions and is therefore particularly important in enabling pathogenic bacteria to invade and survive in host cells. mRNA degradation in E. coli and many other bacteria involves a decay pathway triggered by modification of the 5¢ end of the mRNA transcript by RppH and other enzymes. A better understanding of this pathway could enable the development of new strategies to impede bacterial invasion and survival in hosts. A recently discovered 5¢-end modification of E. coli mRNAs is nucleoside tetraphosphate (Np4) caps, originated from stress-induced “alarmones”, such as Ap4A, present in all domains of life. Despite the identification of capping and decapping enzymes in E. coli, practically nothing is known about mechanisms of cap addition and removal. This proposal details a research plan to elucidate the mechanisms of Np4 capping and decapping and the connection between cellular metabolism and RNA degradation in E. coli. Aim 1 addresses how the RNA polymerase adds Np4A cap precursor to mRNA molecules by using cryogenic electron microscopy, X-ray crystallography, and biochemical experiments to reveal the mechanism and specificity of incorporation. Aim 2 will uncover how the Np4 cap is removed by RppH, using X-ray crystallography and biochemistry to understand the specificity and the catalytic mechanisms of decapping. Aim 3 elucidates the molecular basis of Np4 cap removal by ApaH, using X-ray crystallography and biochemistry to understand the catalytic mechanism and RNA binding rules of this enzyme. Aim 4 reveals a relationship between cellular metabolism and mRNA degradation. This aim uses structure-based genetic uncoupling to identify how the metabolic enzyme DapF affects RNA degradation under various growth conditions. The results of these studies will significantly advance our knowledge of the steps leading to 5¢-end-dependent mRNA degradation and how modulation of this pathway affects the viability of bacteria.