MrpL35 and Mrp7, components of the mitoribosome, play a role in regulating the COX assembly process and influence the molecular environments of the Mss51, Coa3, and Cox14 proteins, chaperones involved in posttranslational assembly events for the COX complex.
When Rhizobium etli CE3 was grown in the presence of Phaseolus vulgaris seed extracts containing anthocyanins, its lipopolysaccharide (LPS) sugar composition was changed in two ways: greatly decreased content of what is normally the terminal residue of the LPS, di-O-methylfucose, and a doubling of the 2-O-methylation of other fucose residues in the LPS O antigen. R. etli strain CE395 was isolated after Tn5 mutagenesis of strain CE3 by screening for mutant colonies that did not change antigenically in the presence of seed extract. The LPS of this strain completely lacked 2-O-methylfucose, regardless of whether anthocyanins were present during growth. The mutant gave only pseudonodules in association with P. vulgaris. Interpretation of this phenotype was complicated by a second LPS defect exhibited by the mutant: its LPS population had only about 50% of the normal amount of O-antigen-containing LPS (LPS I). The latter defect could be suppressed genetically such that the resulting strain (CE395␣395) synthesized the normal amount of an LPS I that still lacked 2-O-methylfucose residues. Strain CE395␣395 did not elicit pseudonodules but resulted in significantly slower nodule development, fewer nodules, and less nitrogenase activity than lps ؉ strains. The relative symbiotic deficiency was more severe when seeds were planted and inoculated with bacteria before they germinated. These results support previous conclusions that the relative amount of LPS I on the bacterial surface is crucial in symbiosis, but LPS structural features, such as 2-O-methylation of fucose, also may facilitate symbiotic interactions.Bacteria that enter into intimate associations with plants and animals, whether pathogenic or mutualistic, respond to signals that are thought to indicate the presence of the host. In the symbiosis between legumes and the bacteria collectively known as rhizobia, one example is the production of Nod factors (lipochitooligosaccharides) by the bacteria in response to flavonoid compounds exuded by the plant (17, 27). Another potential example is the alteration of the lipopolysaccharide (LPS) of Rhizobium etli when grown in the presence of the most potent nod inducers from Phaseolus vulgaris (17), the anthocyanins exuded by germinating seeds (10,22,23). These induced LPS alterations have been detected by the lack of recognition of the altered LPS by certain monoclonal antibodies (23). Modifications of the LPS structure in strains of this and other rhizobial species also have been reported to occur during nodule development and in culture in response to low pH and low oxygen, as well as nod inducers (1,16,18,19,29,31). However, in no case have precise chemical changes been determined, nor is it known whether these induced LPS modifications are important in the development or functioning of the symbiosis.Although little is known of the biological roles of specific structural features of the rhizobial LPS, the importance of wild-type LPS in symbiosis is well established. Studies with mutants of a wide variety of rhizobial s...
The O-antigen polysaccharide (OPS) of Rhizobium etli CE3 lipopolysaccharide (LPS) is linked to the core oligosaccharide via an N-acetylquinovosaminosyl (QuiNAc) residue. A mutant of CE3, CE166, produces LPS with reduced amounts of OPS, and a suppressed mutant, CE166 alpha, produces LPS with nearly normal OPS levels. Both mutants are deficient in QuiNAc production. Characterization of OPS from CE166 and CE166 alpha showed that QuiNAc was replaced by its 4-keto derivative, 2-acetamido-2,6-dideoxyhexosyl-4-ulose. The identity of this residue was determined by NMR and mass spectrometry, and by gas chromatography-mass spectrometry analysis of its 2-acetamido-4-deutero-2,6-dideoxyhexosyl derivatives produced by reduction of the 4-keto group using borodeuteride. Mass spectrometric and methylation analyses showed that the 2-acetamido-2,6-dideoxyhexosyl-4-ulosyl residue was 3-linked and attached to the core-region external Kdo III residue of the LPS, the same position as that of QuiNAc in the CE3 LPS. DNA sequencing revealed that the transposon insertion in strain CE166 was located in an open reading frame whose predicted translation product, LpsQ, falls within a large family of predicted open reading frames, which includes biochemically characterized members that are sugar epimerases and/or reductases. A hypothesis to be tested in future work is that lpsQ encodes UDP-2-acetamido-2,6-dideoxyhexosyl-4-ulose reductase, the second step in the synthesis of UDP-QuiNAc from UDP-GlcNAc.
Rhizobium etli modifies lipopolysaccharide (LPS) structure in response to environmental signals, such as low pH and anthocyanins. These LPS modifications result in the loss of reactivity with certain monoclonal antibodies. The same antibodies fail to recognize previously isolated R. etli mutant strain CE367, even in the absence of such environmental cues. Chemical analysis of the LPS in strain CE367 demonstrated that it lacked the terminal sugar of the wild-type O antigen, 2,3,4-tri-O-methylfucose. A 3-kb stretch of DNA, designated as lpe3, restored wild-type antigenicity when transferred into CE367. From the sequence of this DNA, five open reading frames were postulated. Site-directed mutagenesis and complementation analysis suggested that the genes were organized in at least two transcriptional units, both of which were required for the production of LPS reactive with the diagnostic antibodies. Growth in anthocyanins or at low pH did not alter the specific expression of gusA from the transposon insertion of mutant CE367, nor did the presence of multiple copies of lpe3 situated behind a strong, constitutive promoter prevent epitope changes induced by these environmental cues. Mutations of the lpe genes did not prevent normal nodule development on Phaseolus vulgaris and had very little effect on the occupation of nodules in competition with the wild-type strain.The carbohydrate backbones of lipopolysaccharides (LPSs) of gram-negative bacteria often are highly decorated with substituent chemical groups. Striking examples can be found among Rhizobium spp. and related bacteria (28). For instance, the LPS O chain of Rhizobium etli CE3 (18) is heavily substituted with moieties that should confer hydrophobic character: O-methylations, O-and N-acetylations, and esterification of a repeating carboxyl group (Fig. 1). The hypothetical hydrophobicity is most pronounced at the nonreducing end, where the O-chain repeating units are capped by a terminal deoxysugar in which all of the hydroxyl groups are methylated.A number of interesting questions arise from considering these substituents. One is the mechanism of synthesis; for instance, whether the O-methyl, methyl ester, and O-acetyl groups are added during synthesis of the nucleotide diphosphosugars or after polymerization of the sugar residues. Another issue is the functions of these substituents. With bacteria such as rhizobia, which are known to interact intimately with multicellular hosts, one question is whether such decorations influence these cell-cell interactions.The LPS structures of R. etli and Rhizobium leguminosarum change during the course of infection of their legume hosts and in response to environmental cues, such as plant-released anthocyanins, low pH, and low oxygen concentrations (28, 33). Whether these changes are required for successful bacterialhost interaction remains to be determined. In the case of R. etli CE3, detergent gel electrophoresis and sugar composition analyses indicate that the LPS structure has been altered only slightly after growth i...
We demonstrate here that mitoribosomal protein synthesis, responsible for the synthesis of oxidative phosphorylation (OXPHOS) subunits encoded by mitochondrial genome, occurs at high levels during glycolysis fermentation and in a manner uncoupled from OXPHOS complex assembly regulation. Furthermore, we provide evidence that the mitospecific domain of Mrp7 (bL27), a mitoribosomal component, is required to maintain mitochondrial protein synthesis during fermentation, but is not required under respiration growth conditions. Maintaining mitotranslation under high glucose fermentation conditions also involves Mam33 (p32/gC1qR homolog), a binding partner of Mrp7’s mitospecific domain, and together they confer a competitive advantage for a cell's ability to adapt to respiration-based metabolism when glucose becomes limiting. Furthermore, our findings support that the mitoribosome, and specifically the central protuberance (CP) region, may be differentially regulated and/or assembled, under the different metabolic conditions of fermentation and respiration. Based on our findings, we propose the purpose of mitotranslation is not limited to the assembly of OXPHOS complexes, but also plays a role in mitochondrial signaling critical for switching cellular metabolism from a glycolysis- to a respiratory-based state.
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