## 4 Metazoan 'Stringent-like Response'  
The small alarmone class of synthetases and hydrolases (SAS, SAH), mentioned earlier, is found in prokaryotes and a few lower eukaryotes, but is absent in the metazoo, including all vertebrates such as humans [17]. In fact, as of this writing, there are no reports of a canonical prokaryote-like stringent response. Nonetheless, it is worth drawing the readers' attention to the intriguing discovery [133] of a SpoT sequence ortholog in Drosophila melanogaster (the 'fruit fly') and in cultured human cells, which was named MESH1 and found to play an important role in nutrient deficiency. The response was dubbed a'stringent-like response' by the authors; however, this response turned out to be fundamentally different from the canonical stringent response in prokaryotes. Also noteworthy is that MESH1 appears to be an ortholog of SpoT in sequence and structure (Figure 7), but not in function, and that it has no synthetase activity [134]. In complementary reports, ppGpp was identified as an alarmone in the fruit fly, in human cells [13], and in a few plant chloroplasts [14; 15]. Due to its apparently rare occurrence and under-studied mechanism, the'stringent-like response' is summarized here only briefly. When expressed recombinantly, the MESH1 protein functionally partially complemented SpoT-deficiency in bacteria and suppressed RelA-induced inhibition of cell growth, the mechanism of which has not been resolved [11]. Drosophila with deleted Mesh1 gene exhibited stunted body growth and sensitivity to starvation. As in the prokaryotic stringent control, when starved for amino acids, the Mesh1-null fly underwent down-regulation of genes related to replication and translation and upregulation of stress-inducible genes. Studies of the Mesh1 mechanism of action revealed its major distinction from the canonical stringent response in prokaryotes. First, Drosophila as well as human MESH1 was found to dephosphorylate NADPH (Nicotinamide Adenine Dinucleotide Phosphate, reduced form) [134; 135; 136] to NADH. Thus, MESH1, unlike SpoT/RelA, exhibits a broader substrate specificity, being able to dephosphorylate both ppGpp and NADPH [135]. Second, loss of MESH1, and the resultant loss of the NADPH phosphatase activity of the cell, appears to protect cells from untoward stress, specifically from 'ferrotposis' [135; 136]. Recently recognized as a new form of cell death, triggered by iron-dependent lipid peroxidation, ferrotposis was later found to be induced by a variety of stressors, including endoplasmic reticular stress (ER stress), a complex signaling network activated by misfolded or unfolded proteins [137; 138]. The apparent overlap between cellular responses to various forms of stress has led to the concept of the 'integrated stress response' [139]. Meanwhile, phylogenetic sequence analysis led to the discovery of Mesh1-like genes in several bacterial genera, including Cellulophaga, Eryththoracter, and Flavobacterium [140], coding for SpoT orthologs that contained hydrolytic domains. Interestingly, several species of extremophiles that thrive in harsh habitats, such as extremely high or low temperatures and in saline marine waters, belong to these genera [141]. These extremophiles, especially many Flavobacteria, also encode a unique family of immunophilins, named 'dual-family immunophilin' (DFI) [142; 143]. The DFI, sometimes referred as 'foldase', promotes or maintains native folding of proteins, particularly needed in extreme environments in which proteins would otherwise misfold and/or denature [142; 143]. With this premise, we would like to propose that MESH1 (or a close ortholog) and the ER stress response serve as major stress cues in the extremophiles, which awaits experimental evidence. homologous regions, particularly considering that the central dissimilar sequence (underlined in Panel A) contained unaligned loops that incurred a penalty in RMSD [144; 145].