Azotobacter vinelandii aldehyde dehydrogenase regulated by sigma(54): role in alcohol catabolism and encystment

Expression of the sRNAs CrcZ and CrcY modulate the strength of carbon catabolite repression under diazotrophic or non-diazotrophic growing conditions in Azotobacter vinelandii

Azotobacter vinelandii is a nitrogen-fixing bacterium of the Pseudomonadaceae family that prefers the use of organic acids rather than carbohydrates. Thus, in a mixture of acetate-glucose, glucose is consumed only after acetate is exhausted. In a previous work, we investigated the molecular basis of this carbon catabolite repression (CCR) process under diazotrophic conditions. In the presence of acetate, Crc-Hfq inhibited translation of the gluP mRNA, encoding the glucose transporter in A. vinelandii. Herein, we investigated the regulation in the expression of the small non-coding RNAs (sRNAs) crcZ and crcY, which are known to antagonize the repressing activity of Hfq-Crc. Our results indicated higher expression levels of the sRNAs crcZ and crcY under low CCR conditions (i.e. glucose), in relation to the strong one (acetate one). In addition, we also explored the process of CCR in the presence of ammonium. Our results revealed that CCR also occurs under non-diazotrophic conditions as we detected a hierarchy in the utilization of the supplied carbon sources, which was consistent with the higher expression level of the crcZ/Y sRNAs during glucose catabolism. Analysis of the promoters driving transcription of crcZ and crcY confirmed that they were RpoN-dependent but we also detected a processed form of CrcZ (CrcZ*) in the RpoN-deficient strain derived from a cbrB-crcZ co-transcript. CrcZ* was functional and sufficient to allow the assimilation of acetate.

Sigma Factor Regulated Cellular Response in a Non-solvent Producing Clostridium beijerinckii Degenerated Strain: A Comparative Transcriptome Analysis

Clostridium beijerinckii DG-8052, derived from NCIMB 8052, cannot produce solvent or form spores, a phenomenon known as degeneration. To explore the mechanisms of degeneration at the gene level, transcriptomic profiles of the wild-type 8052 and DG-8052 strains were compared. Expression of 5168 genes comprising 98.6% of the genome was assessed. Interestingly, 548 and 702 genes were significantly up-regulated in the acidogenesis and solventogenesis phases of DG-8052, respectively, and mainly responsible for the phosphotransferase system, sugar metabolic pathways, and chemotaxis; meanwhile, 699 and 797 genes were significantly down-regulated, respectively, and mainly responsible for sporulation, oxidoreduction, and solventogenesis. The functions of some altered genes, including 286 and 333 at the acidogenesis and solventogenesis phases, respectively, remain unknown. Dysregulation of the fermentation machinery was accompanied by lower transcription levels of glycolysis rate-limiting enzymes (pfk and pyk), and higher transcription of cell chemotaxis genes (cheA, cheB, cheR, cheW, and cheY), controlled mainly by σ⁵⁴ at acidogenesis. Meanwhile, abnormal spore formation was associated with repressed spo0A, sigE, sigF, sigG, and sigK which are positively regulated by σ⁷⁰, and correspondingly inhibited expression of CoA-transferase at the solventogenesis phase. These findings indicated that morphological and physiological changes in the degenerated Clostridium strain may be related to altered expression of sigma factors, providing valuable targets for strain development of Clostridium species.

Sigma factor RpoS controls alkylresorcinol synthesis through ArpR, a LysR-type regulatory protein, during encystment of Azotobacter vinelandii

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.

Encystment and alkylresorcinol production by Azotobacter vinelandii strains impaired in poly-beta-hydroxybutyrate synthesis

The lipids poly-beta-hydroxybutyrate (PHB) and alkylresorcinols are the major metabolic products of Azotobacter vinelandii cysts. Cysts are formed in less than 0.01% of late stationary phase cells grown on sucrose. Culturing vegetative cells in n-butanol or beta-hydroxybutyrate induces encystment. After induction of encystment, PHB rapidly accumulates in large granules. Then, the cells begin the synthesis of alkylresorcinols that replace the phospholipids in the membranes and are components of the exine, the outer layer of the cyst envelope. Vegetative cells do not synthesize alkylresorcinols. We report here the effect of mutations in the phbBAC operon, coding for the enzymes of the PHB biosynthetic pathway, on the synthesis of alkylresorcinols and cyst formation. The phb mutations did not impair the capacity to form mature cysts. However, the cysts formed by these strains posses a thicker exine layer and a higher content of alkylresorcinols than the cysts formed by the wild-type strain. A blockage of PHB synthesis caused by phb mutations resulted in the synthesis of alkylresorcinols and encystment even under non-inducing conditions. We propose that, as a consequence of the blockage in the PHB biosynthetic pathway, the acetyl-CoA and reducing power pools are increased causing the shift to lipid metabolism required for the synthesis of alkylresorcinols and cyst formation.