Calcium regulates the mycophagous ability of Burkholderia gladioli strain NGJ1 in a type III secretion system-dependent manner

BACKGROUND

A rice associated bacterium Burkholderia gladioli strain NGJ1 demonstrates mycophagy, a phenomenon wherein bacteria feed on fungi. Previously, we have reported that NGJ1 utilizes type III secretion system (T3SS) to deliver a prophage tail-like protein (Bg_9562) into fungal cells to establish mycophagy.

RESULTS

In this study, we report that calcium ion concentration influences the mycophagous ability of NGJ1 on Rhizoctonia solani, an important fungal pathogen. The calcium limiting condition promotes mycophagy while high calcium environment prevents it. The expression of various T3SS apparatus encoding genes of NGJ1 was induced and secretion of several potential T3SS effector proteins (including Bg_9562) into extracellular milieu was triggered under calcium limiting condition. Using LC-MS/MS proteome analysis, we identified several calcium regulated T3SS effector proteins of NGJ1. The expression of genes encoding some of these effector proteins was upregulated during mycophagous interaction of NGJ1 with R. solani. Further, mutation of one of these genes (endo-β-1, 3- glucanase) rendered the mutant NGJ1 bacterium defective in mycophagy while complementation with full length copy of the gene restored its mycophagous activity.

CONCLUSION

Our study provides evidence that low calcium environment triggers secretion of various T3SS effectors proteins into the extracellular milieu and suggests the importance of cocktail of these proteins in promoting mycophagy.

The extracellular endo-β-1,4-xylanase with multidomain from the extreme thermophile Caldicellulosiruptor lactoaceticus is specific for insoluble xylan degradation

BACKGROUND

The extremely thermophilic bacterium Caldicellulosiruptor lactoaceticus can degrade and metabolize untreated lignocellulosic biomass containing xylan. The mechanism of the bacterium for degradation of insoluble xylan in untreated biomass has not been revealed.

RESULTS

In the present study, the only annotated extracellular endo-β-1,4-xylanase (Xyn10B) with multidomain structures in C. lactoaceticus genome was biochemically characterized. Xyn10B contains three N-terminal consecutive family 22 carbohydrate-binding modules (CBMs), one GH10 catalytic domain (CD), two family 9 CBMs and two S-layer homology (SLH) modules in the C-terminal. CBM22a shares 27.1% and 27.2% sequence homology with CBM22b and CBM22c, respectively. The sequence homology between two CBM9 s and two SLHs is 26.8% and 25.6%, respectively. To elucidate the effect of multiple domains on the enzymatic properties of Xyn10B, the truncated variants of which (Xyn10B-TM1: CBM22a-CBM22b-CBM22c-CD10; Xyn10B-TM2: CBM22c-CD10; Xyn10B-TM3: CBM22c-CD10-CBM9a; and Xyn10B-TM4: CD10-CBM9a) were separately reconstructed, recombinantly expressed and biochemically characterized. Enzymatic properties studies showed that the optimal temperature for all four Xyn10B truncations was 65 °C. Compared to Xyn10B-TM3 and Xyn10B-TM4, Xyn10B-TM1 and Xyn10B-TM2 had higher hydrolytic activity, thermostability and affinity on insoluble substrates. It is noteworthy that Xyn10B-TM1 and Xyn10B-TM2 have higher enzymatic activity on insoluble xylan than the soluble counterparts, whereas Xyn10B-TM3 and Xyn10B-TM4 showed opposite characteristics. The kinetic parameters analysis of Xyn10B-TM1 on xylan showed V _max was 5740, 1300, 1033, and 3925 U/μmol on insoluble oat spelt xylan (OSX), soluble beechwood xylan (BWX), soluble sugar cane xylan (SCX), and soluble corncob xylan (CCX), respectively. The results indicated that CBM22s especially CBM22c promoted the hydrolytic activity, thermostability and affinity on insoluble substrates of the Xyn10B truncations. The functions of CBM22, CBM9, CD and SLH are different, while contribute synergetically to the thermostability, protein structure integrity, substrate binding, and high hydrolytic activity on insoluble xylan of untreated lignocellulosic biomass. The domains of CBM22, CBM9, CD and SLH have different characteristics, which synergistically promote the thermostability, protein structure integrity, affinity on insoluble substrates and enzymatic activity properties of Xyn10B.

CONCLUSIONS

The extracellular endo-β-1,4-xylanase with multidomain structures of CBM, CD and SLH promote the biodegradation of insoluble xylan in untreated lignocellulosic biomass by thermophilic C. lactoaceticus.

Cloning and Characterization of an Endoglucanase Gene from Actinomyces sp. Korean Native Goat 40

A gene from Actinomyces sp. Korean native goat (KNG) 40 that encodes an endo-β-1,4-glucanase, EG1, was cloned and expressed in Escherichia coli (E. coli) DH5α. Recombinant plasmid DNA from a positive clone with a 3.2 kb insert hydrolyzing carboxyl methyl-cellulose (CMC) was designated as pDS3. The entire nucleotide sequence was determined, and an open-reading frame (ORF) was deduced. The ORF encodes a polypeptide of 684 amino acids. The recombinant EG1 produced in E. coli DH5α harboring pDS3 was purified in one step using affinity chromatography on crystalline cellulose and characterized. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis/zymogram analysis of the purified enzyme revealed two protein bands of 57.1 and 54.1 kDa. The amino terminal sequences of these two bands matched those of the deduced ones, starting from residue 166 and 208, respectively. Putative signal sequences, a Shine–Dalgarno-type ribosomal binding site, and promoter sequences related to the consensus sequences were deduced. EG1 has a typical tripartite structure of cellulase, a catalytic domain, a serine-rich linker region, and a cellulose-binding domain. The optimal temperature for the activity of the purified enzyme was 55°C, but it retained over 90% of maximum activity in a broad temperature range (40°C to 60°C). The optimal pH for the enzyme activity was 6.0. Kinetic parameters, K_m and V_max of rEG1 were 0.39% CMC and 143 U/mg, respectively.

A simple and efficient diffusion technique for assay of endo β-1,4-xylanase activity

Endo-β-1, 4-xylanases is thought to be of great significance for several industries namely paper, pharmaceuticals, food, feed etc. in addition to better utilization of lignocellulosic biomass. The present investigation was aimed to develop an easy, simple and efficient assay technique for endo-β-1, 4-xylanases secreted by the aerobic fungi. Under the proposed protocol, 9 g/L xylan containing agar was prepared in 100 mM phosphate buffer at different pH (4.5, 5.5 and 6.5). The sterilized xylan agar was dispensed in 90 mm petri dishes. 100 µl of culture supernatant of 12 fungal isolates was added to the wells and left overnight at 31±1⁰C. The petri dishes were observed for zone of clearance by naked eye and diameter was measured. Congo red solution (1 g/L) was applied over the petri dishes as per the established protocol and thereafter plates were flooded with 1M Sodium chloride solution for the appearance of zone of clearance. The diameter for zone of clearance by the proposed method and the established protocol was almost identical and ranged from 21 to 42 mm at different pH depending upon the activity of endo-β-1, 4-xylanases. Change of pH towards alkaline side enabled similar or marginal decrease of diameter for the zone of clearance in most of the fungal isolates. The specific activities of these fungal isolates varied from 1.85 to 11.47 IU/mg protein. The present investigation revealed that the proposed simple diffusion technique gave similar results as compared to the established Congo red assay for endo-β-1, 4-xylanases. Moreover, the present technique avoided the cumbersome steps of staining by Congo red and de-staining by sodium chloride.