Moreover, the Carboxy-terminal HD domain of the E. coli tRNA nucleotidyltransferase has a metal-independent phosphodiesterase activity toward 2′, 3′cAMP [35]. Thus, the fact that SpdA displays metal-independent 2′, 3′cNMP-phosphodiesterase activity is not completely unprecedented. Mass spectrometric measurements performed under mild ionization conditions buy INK 128 also pointed out that the well-defined

monomeric form of the protein did not present any demetallation. The 2′, 3′cNMP substrate specificity of SpdA leaves the question of 3′, 5′cAMP turnover intact. One option would be to identify a 3′, 5′cNMP PDE among the 14 other S. meliloti proteins containing the IPR004843 domain. Another, non-exclusive, possibility would be a regulation of 3′, 5′cAMP homeostasis by secretion rather than by degradation [36]. Possible biological functions for SpdA Very little is known about the origin, role and fate of 2′, 3′cyclic nucleotides. One documented origin is RNA degradation and physiological or stressful conditions may indeed lead 2′, 3′cNMPs to accumulate

in bacteria. We are not aware of any other origin such as, for example, isomerization of corresponding 3’, 5’ cyclic nucleotides. In this context, SpdA may serve at least three different, non-exclusive, functions: a metabolic function, a detoxifying function and a role in preventing cross talk with 3′, 5′cAMP signaling. OSI906 Although S. meliloti likely metabolized exogenous 2′, 3′cAMP (See Additional file 7), spdA was not critical for this since the mutant grew indistinctly from wild-type under these conditions. 2′, 3′cAMP

was recently reported to be a toxic compound in kidney cells, that opens mitochondria permeability transition pores thus leading to Protein tyrosine phosphatase a pre-apoptotic and necrotic stage [37]. We thus considered whether SpdA may counteract a toxic effect of 2′, 3′cNMPs in S. meliloti. However the unaltered growth characteristics of the spdA mutant as compared to wild-type in various growth (including the presence of exogenous 2′, 3′cAMP) and stress conditions (see Additional file 7) did not give support to this possibility. A third possibility would be SpdA preventing cross-talk between 2′, 3′cyclic nucleotides and 3′, 5′cAMP signaling. Several lines of evidence are in favor of this possibility: (i) the evolutionary-conserved physical location of spdA in close proximity to cyaD1, clr and the target gene smc02178 in all the sequenced strains of Sinorhizobium meliloti, Sinorhizobium saheli and Sinorhizobium fredii (https://​www.​genoscope.​cns.