Microbial secondary metabolites are low molecular mass products, not essential for growth of the producing cultures, but very important for human health. They include antibiotics, antitumor agents, cholesterol-lowering drugs, and others. They have unusual structures and are usually formed during the late growth phase of the producing microorganisms. Its synthesis can be influenced greatly by manipulating the type and concentration of the nutrients formulating the culture media. Among these nutrients, the effect of the carbon sources has been the subject of continuous studies for both, industry and research groups. Different mechanisms have been described in bacteria and fungi to explain the negative carbon catabolite effects on secondary metabolite production. Their knowledge and manipulation have been useful either for setting fermentation conditions or for strain improvement. During the last years, important advances have been reported on these mechanisms at the biochemical and molecular levels. The aim of the present review is to describe these advances, giving special emphasis to those reported for the genus Streptomyces.
In Streptomyces coelicolor, the sco2127 gene is located upstream of the gene encoding for glucose kinase. This region restores sensitivity to carbon catabolite repression (CCR) of Streptomyces peucetius var. caesius mutants, resistant to 2-deoxyglucose (Dog(R)). In order to search for the possible mechanisms behind this effect, sco2127 was overexpressed and purified for protein-protein interaction studies. SCO2127 was detected during the late growth phase of S. coelicolor grown in a complex media supplemented with 100 mM glucose. Pull-down assays using crude extracts from S. coelicolor grown in the same media, followed by far-western blotting, allowed detection of two proteins bound to SCO2127. The proteins were identified by MALDI-TOF mass spectrometry as SCO5113 and SCO2582. SCO5113 (BldKB) is a lipoprotein ABC-type permease (∼66 kDa) involved in mycelium differentiation by allowing the transport of the morphogenic oligopeptide Bld261. SCO2582, is a putative membrane metalloendopeptidase (∼44 kDa) of unknown function. In agreement with the possible role of SCO2127 in mycelium differentiation, delayed aerial mycelium septation and sporulation was observed when S. coelicolor A3(2) was grown in the presence of elevated glucose concentrations (100 mM), an effect not seen in a Δ-sco2127 mutant derived from it. We speculate that SCO2127 might represent a key factor in CCR of mycelium differentiation by interacting with BldKB.
Streptomyces coelicolor mutants resistant to 2-deoxyglucose are insensitive to carbon catabolite repression (CCR). Total reversion to CCR sensitivity is observed by mutant complementation with a DNA region harboring both glucose kinase glkA gene and the sco2127 gene. The sco2127 is located upstream of glkA and encodes a putative protein of 20.1 kDa. In S. coelicolor, actinorhodin production is subject to glucose repression. To explore the possible involvement of both SCO2127 and glucose kinase (Glk) in the glucose sensitivity of actinorhodin production, this effect was evaluated in a wild-type S. coelicolor A3(2) M145 strain and a sco2127 null mutant (Δsco2127) derived from this wild-type strain. In comparison with strain M145, actinorhodin production by the mutant was insensitive to glucose repression. Under repressive conditions, only minor differences were observed in glucose utilization and Glk production between these strains. SCO2127 was detected mainly during the first 36 h of fermentation, just before the onset of antibiotic production, and its synthesis was not related to a particular carbon source. The glucose sensitivity of antibiotic production was restored to wild-type phenotype by transformation with an integrative plasmid containing sco2127. Our results support the hypothesis that SCO2127 is a negative regulator of actinorhodin production and suggest that the effect is independent of Glk.
Contextualization: Pseudomonas aeruginosa is capable of producing biosurfactants which have many uses in bioremediation and the production of antiviral, antibacterial, antiparasitic, sporicidal and antifungal agents, among others. Knowledge gap: This study describes the production of mono and di-rhamnolipid biosurfactants by P. aeruginosa strains isolated from Zea mays rhizosphere and composts in the state of Guerrero, Mexico. Purpose: The overall aims were to investigate biosurfactant, pyocyanin production, and tolerance to heavy metals and antimicrobial activity capacity than biosurfactants produced from P. aeruginosa strains from corn rhizosphere and compost in Mexico. Methodology: Biosurfactant production was determined based hemolysis on blood agar, blue halos in CTAB-Methylene blue agar, drop collapse test and production of foam on PPGAS broth, the emulsion index (IE24) and antibacterial capacity. The strains were identified by sequence of the 16S rDNA gene and their resistance to heavy metals were also evaluated. Results and conclusions: Two strains isolated from Zea mays rhizosphere (PAM8, PAM9) were the best biosurfactant producers and their extracts showed antimicrobial activity against Grampositive and Gramnegative bacteria. PAM8 and PAM9 showed >30% of cellular hydrophobicity to hydrocarbons, and were capable of emulsifying toluene, cyclohexane, petroleum, diesel and oils. All strains showed the same profile of heavy metal tolerance (As5+ >As3+ >Zn2+ >Pb2+ >Fe3+ >Cd2+ >Cu2+ >Cr6+ in concentrations of 20, 10, 10, 6, 4, 4, 2 and 2 mM., respectively). The isolation of biosurfactant-producing and heavy-metal tolerant bacteria from Zea mays rhizosphere and compost in Guerrero demonstrates the capacity for this region to harbor potentially important microbial strains for industrial or bioremediation applications.
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