# Riboregulation in Nitrogen-Fixing Endosymbiotic Bacteria
## 4 Assigning Functions to Rhizbial _Trans_-sRNAs
### Nutrient Uptake and Metabolism
AbcR1 is accumulated in exponentially growing bacteria and in the nodule invasion zone II, which accommodates undifferentiated actively dividing rhizobia. In contrast, AbcR2 is highly induced in late stationary phase and other stress conditions, but not in nodules [62]. The molecular determinants of the transcriptional regulation of these \(S\). _meliloti_ sRNAs are unknown. Analysis of the \(abcR2\) promoter shows high conservation of motifs recognized by the heat-shock \(\sigma\) factor RpoH, whereas the _Brucella abcR1_ is directly and positively regulated by the LsrB homolog VltR, encoded by the neighbor gene. Interestingly, while VltR is involved in virulence, LsrB is critical for the successful symbiosis in alfalfa plants by suppression of host defense responses that contribute to nodule premature senescence [64; 65; 66; 67]. Neither AbcR1 nor AbcR2 are required for the competitive and efficient modulation of \(S\). _meliloti_ on alfalfa roots or free-living growth. However, AbcR homologs in _Brucella_ or _Agrobacteriumium_ are involved in virulence [61]. Notably, whereas \(S\). _meliloti_ _liv_ ABC system has no contribution to symbiosis, its \(bra\) homolog in \(R\). _leguminosarum_ is needed for a Fix+ phenotype in pea [68; 69; 70]. Therefore, it cannot be ruled out a symbiotic relevance of AbcR sRNAs in other rhizobia-host interactions. Proper regulation of N and carbon (C) metabolism is also required for successful bacteroid differentiation and N fixation. C storage in nodules occurs in the form of polyhydroxybutyrate (PHB) granules which are produced during the invasion process and degraded during bacteroid differentiation. PHB is likely used as a C source at this stage, being critical for symbiotic performance [71]. MmgR(makes more granules regulator) is an Hfq-dependent _S. melilioti_ 77 nt-long _trans_-sRNA of the \(\alpha\)r8 RNA family (Table 1) linked to PHB metabolism. MmgR homologs have been also identified in _S. fredii_, _R. leguminosarum_bv. _viciae_ and _R. etli_[45]. The predicted MmgR secondary structure shows three SLs and partial homology to SuhB sRNA. An aSD motif within the first loop is predicted to be involved in targeting, but no direct mRNA target of this sRNA has been described to date [45]. MmgR accumulates preferentially upon entry of bacteria into stationary phase when grown in defined minimal medium. In addition, it is induced by nitrate-limiting conditions and repressed by addition of tryptone or aa to the medium. The transcription of \(mmgR\) is induced by the global N regulator NtrC, while it is repressed by AntiA, a regulator of C flow. As revealed by the loss-of-function phenotype, MmgR preserves synthesis of both PHB-related phasin proteins (PhaP1 and PhaP2) under conditions of C surplus, thereby limiting intracellular accumulation of PHB granules. Together, these data suggest that \(mmgR\) expression depends on cellular N and C status [72; 73]. However, lack of the PHB synthase coding gene \(phbC\) has different effect depending on rhizbial species. Therefore, MmgR may have a different impact in other rhizobia that is worth to investigate [74; 75; 76; 70].