During encystment of Azotobacter vinelandii , a family of alkylresorcinols (ARs) and alkylpyrones (APs) are synthesized. In the mature cyst, these lipids replace the membrane phospholipids and are also components of the layers covering the cyst. In this study, A. vinelandii strains unable to synthesize ARs were isolated after mini-Tn 5 mutagenesis. Cloning and nucleotide sequencing of the affected loci revealed the presence of the transposons within the arsA gene of the previously reported arsABCD gene cluster, which encodes a type I fatty acid synthase. A mutant strain (SW-A) carrying an arsA mutation allowing transcription of arsBCD was constructed and shown to be unable to produce ARs, indicating that the ArsA protein is essential for the synthesis of these phenolic lipids. Transcription of arsA was induced 200-fold in cells undergoing encystment, but only 14-fold in aged cultures of A. vinelandii , in accordance with AR synthesis and cyst formation percentages under the two conditions. Although it was previously reported that the inactivation of arsB abolishes AR synthesis and results in a failure in encystment, the arsA mutants were able to form cysts resistant to desiccation. These data indicate that ARs play a structural role in the exine layer of the cysts, but they are not essential for either cyst formation or for desiccation resistance.

Azotobacter vinelandii is a bacterium which undergoes a differentiation process leading to the formation of metabolically dormant cysts. During the encystment process, A. vinelandii produces alkylresorcinol lipids (ARs) that replace the membrane phospholipids and are also components of the layers covering the cyst. The synthesis of ARs in A. vinelandii has been shown to occur by the activity of enzymes encoded by the arsABCD operon, which is expressed only during the differentiation process. Also, the production of ARs has been shown to be dependent on the stationary-phase sigma factor RpoS, which is also implicated in the control of the synthesis of other cyst components (i.e., alginate and poly-β-hydroxybutyrate). In this study, we identified ArpR, a LysR-type transcriptional regulator expressed only during encystment that positively regulates arsABCD transcription. We show that this activation is dependent on acetoacetyl-coenzyme A (acetoacetyl-CoA), which might provide a metabolic signal for encystment. We also show that RpoS regulates arsABCD expression through the control of arpR transcription.

Upon encystment induction, Azotobacter vinelandii produces the phenolic lipids alkylresorcinols (ARs) that are structural components of the cysts. The enzymes responsible for the ARs synthesis are encoded in the arsABCD operon, whose expression is activated by ArpR. The transcription of arpR is initiated from an RpoS dependent promoter. The nitrogen-related phosphotransferase system (PTS^Ntr) is a global regulatory system present in Gram negative bacteria. It comprises the EI^Ntr, NPr and EIIA^Ntr proteins encoded by ptsP, ptsO and ptsN genes respectively. These proteins participate in a phosphoryl-group transfer from phosphoenolpyruvate to protein EIIA^Ntr via the phosphotransferases EI^Ntr and NPr. In A. vinelandii, the non-phosphorylated form of EIIA^Ntr was previously shown to repress the synthesis of poly-ß-hydroxybutyrate. In this work, we show that PTS^Ntr also regulates the synthesis of ARs. In a strain that carries unphosphorylated EIIA^Ntr, the expression of arpR was reduced, while synthesis of ARs and transcription of arsA were almost abrogated. The expression of arpR from an RpoS-independent promoter in this strain restored the ARs synthesis. Taken together these results indicate that unphosphorylated EIIA^Ntr negatively affects activation of arpR transcription by RpoS.

Mine tailings contain high concentrations of heavy metals such as As, Pb, Cu, Mn, and Fe, which are detrimental to the health of humans and the environment. In tailings at the El Fraile mine in Guerrero, Mexico, some plant species are apparently tolerant of heavy metals and can be found growing in the tailings. These plants could be associating with heavy metal-tolerant bacteria that promote plant growth and improve biomass production, and these bacteria could be a useful alternative for bacteria-assisted phytoremediation. The objective of this study was to isolate bacteria detected in the mine tailings at El Fraile-Taxco, focusing on those in the soil from the rhizosphere, the inner tissue of the root, leachate, and water, which have the potential to promote plant growth. The ability of the isolated bacteria to promote plant growth was evaluated in vitro. Of the 151 morphotypes isolated, 51% fix nitrogen, 12% dissolve phosphates, and 12%, 39.7%, and 48.3% produce indole acetic acid, gibberellins, and siderophores, respectively. In addition, 66.7% were observed to produce lytic enzymes, such as proteases, celluloses, lipases, esterases, and amylases, which exhibited activity against Fusarium, Aspergillus, and Colletotrichum. The use of 16S rRNA analysis led to the identification of the bacterial genera Chryseobacterium, Bacillus, Pseudomonas, Mycobacterium, Staphylococcus, Curtobacterium, Enterobacter, Agrobacterium, Ochrobactrum, Serratia, Stenotrophomonas, and Acinetobacter. The bacteria isolated from the rhizosphere exhibited the greatest ability to fix nitrogen and produced indole acetic acid, gibberellins, siderophore, and lytic enzymes. In addition, the isolates collected from the soil samples demonstrated ability to solubilize phosphate.