Eukaryotic cells encode AMP-lysine (AMP-N-⑀-(N-␣acetyl lysine methyl ester) 5-phosphoramidate) hydrolases related to the rabbit histidine triad nucleotidebinding protein 1 (Hint1) sequence. Bacterial and archaeal cells have Hint homologs annotated in a variety of ways, but the enzymes have not been characterized, nor have phenotypes been described due to loss of enzymatic activity. We developed a quantitative 31 P NMR assay to determine whether Escherichia coli possesses an adenosine phosphoramidase activity. Indeed, soluble lysates prepared from wild-type laboratory E. coli exhibited activity on the model substrate adenosine 5-monophosphoramidate (AMP-NH 2 ). The E. coli Hint homolog, which had been comprehensively designated ycfF and is here named hinT, was cloned, overexpressed, purified, and characterized with respect to purine nucleoside phosphoramidate substrates. Bacterial hinT was several times more active than human or rabbit Hint1 on five model substrates. In addition, bacterial and mammalian enzymes preferred guanosine versus adenosine phosphoramidates as substrates. Analysis of the lysates from a constructed hinT knock-out strain of E. coli demonstrated that all of the cellular purine nucleoside phosphoramidase activity is due to hinT. Physiological analysis of this mutant revealed that the loss of hinT results in failure to grow in media containing 0.75 M KCl, 0.9 M NaCl, 0.5 M NaOAc, or 10 mM MnCl 2 . Thus, cation-resistant bacterial cell growth may be dependent on the hydrolysis of adenylylated and/or guanylylated phosphoramidate substrates by hinT.

Eukaryotic initiation factor 2 (eIF2) is a central regulator of translational initiation in times of growth and times of stress. Here we discovered three new conserved regulators of eIF2 in Saccharomyces cerevisiae. cdc123, homolog of mammalian D123, is a new cell division cycle mutant with a G 2 delay at permissive temperature and a terminal, mating-proficient G 1 arrest point. Cdc123 protein is regulated by nutrient availability. CHF1 and CHF2, homologs of mammalian checkpoint forkhead associated with RING genes, are required for G 2 delay and G 1 arrest of cdc123-4 and promote G 1 delay when overexpressed. Cell cycle delaying activity and the natural instability of Chf1 and Chf2 depend on the integrity of both domains and association with Cdc123. Genetic analysis maps the Chf1 forkhead associated domainbinding site to the conserved Thr-274 of Cdc123, suggesting that mammalian D123 is a key target of Chfr. Gcd11, the ␥ subunit of eIF2, is an additional Cdc123-interacting protein that is an essential target of the Cdc123 cell cycle promoting and Chf cell cycle arresting activity whose abundance is regulated by Cdc123, Chf1, and Chf2. Loss of cdc123 activity promotes Chf1 and Chf2 accumulation and Gcd11 depletion, accounting for the essentiality of Cdc123. The data establish the Cdc123-Chf-Gcd11 axis as an essential pathway for nutritional control of START that runs parallel to the Tor-Gcn2-Sui2 system of translational control.