Rahnella sp., a Dominant Symbiont of the Core Gut Bacteriome of Dendroctonus Species, Has Metabolic Capacity to Degrade Xylan by Bifunctional Xylanase-Ferulic Acid Esterase

Abstract

Rahnella sp. ChDrAdgB13 is a dominant member of the gut bacterial core of species of the genus Dendroctonus, which is one of the most destructive pine forest bark beetles. The objectives of this study were identified in Rahnella sp. ChDrAdgB13 genome the glycosyl hydrolase families involved in carbohydrate metabolism and specifically, the genes that participate in xylan hydrolysis, to determine the functionality of a putative endo-1,4-β-D-xylanase, which results to be bifunctional xylanase–ferulic acid esterase called R13 Fae and characterize it biochemically. The carbohydrate-active enzyme prediction revealed 25 glycoside hydrolases, 20 glycosyl transferases, carbohydrate esterases, two auxiliary activities, one polysaccharide lyase, and one carbohydrate-binding module (CBM). The R13 Fae predicted showed high identity to the putative esterases and glycosyl hydrolases from Rahnella species and some members of the Yersiniaceae family. The r13 fae gene encodes 393 amino acids (43.5 kDa), containing a signal peptide, esterase catalytic domain, and CBM48. The R13 Fae modeling showed a higher binding affinity to ferulic acid, α-naphthyl acetate, and arabinoxylan, and a low affinity to starch. The R13 Fae recombinant protein showed activity on α-naphthyl acetate and xylan, but not on starch. This enzyme showed mesophilic characteristics, displaying its optimal activity at pH 6.0 and 25°C. The enzyme was stable at pH from 4.5 to 9.0, retaining nearly 66–71% of its original activity. The half-life of the enzyme was 23 days at 25°C. The enzyme was stable in the presence of metallic ions, except for Hg2+. The products of R13 Fae mediated hydrolysis of beechwood xylan were xylobiose and xylose, manifesting an exo-activity. The results suggest that Rahnella sp. ChDrAdgB13 hydrolyze xylan and its products could be assimilated by its host and other gut microbes as a nutritional source, demonstrating their functional role in the bacterial-insect interaction contributing to their fitness, development, and survival.

A polyphasic taxonomy analysis reveals the presence of an ecotype of Rahnella contaminans associated with the gut of Dendroctonus-bark beetles

“…In brief, this study shows that Rahnella isolates associated with the gut of Dendroctonus -bark beetles are an ecotype of R. contaminans . This ecotype is a recurrent, dominant, and persistent symbiont in the gut, which plays a fundamental ecological role in the benefit of these bark beetles, due to its capacity to hydrolyze esters, lipids, and xylans ( Briones-Roblero et al, 2017b ; Pineda-Mendoza et al, 2022 ), as well as recycle uric acid ( Morales-Jiménez et al, 2013 ), and degrade or transform different toxic monoterpenes for insects ( Boone et al, 2013 ; Xu et al, 2015 , 2016 ).…”

“…( Hernández-García et al, 2017 ), and it has been persistently reported in all developmental stages of Dendroctonus rhizophagus ( Briones-Roblero et al, 2017a ). Furthermore, it has been shown that Rahnella strains from bark beetles play a fundamental role in the digestive and detoxification processes, since they are capable of degrading different substrates such as esters, lipids, and xylan ( Briones-Roblero et al, 2017b ; Pineda-Mendoza et al, 2022 ), recycling uric acid ( Morales-Jiménez et al, 2013 ), and degrading or transforming different monoterpenes of the host trees of these insects ( Boone et al, 2013 ; Xu et al, 2015 , 2016 ).…”

Insect Bacteriocytes: Adaptation, Development, and Evolution

“…Nearly all of the insect species harboring bacteriocyte symbionts live on nutritionally poor or unbalanced diets, e.g., plant sap, animal blood, and wood (14). The bacteriocyte symbionts can help insects utilize such diets by providing specific nutrients that the insect cannot synthesize, such as essential amino acids (EAAs), B vitamins, and sterols, and by degrading complex plant polysaccharides (16,82,90). The nutritional symbionts in bacteriocytes are indispensable for the normal growth, development, and reproduction of host insects (14,16,32).…”

An insight into microbial sources, classification, and industrial applications of xylanases: A rapid review

“…Structurally, xylan is not a simple polymer, as it is not only composed of homopolymeric or linear backbone but may also contain branches or side chains, which adds heteropolymeric character to its backbone 29,30 . Monomers of β‐D‐xylopyranosyl are linked in a linear fashion to form homopolymeric long chains, whereas addition of variety of side chains such as o‐acetyl, 4‐O‐methyl‐D‐glucurono‐ pyranosyl, α‐L‐arabinofuranosyl, feruloyl or p‐coumaroyl, and so forth contribute to the formation of heteropolymeric structure 29,31,32 …”

“…29,30 Monomers of β-D-xylopyranosyl are linked in a linear fashion to form homopolymeric long chains, whereas addition of variety of side chains such as o-acetyl, 4-O-methyl-D-glucurono-pyranosyl, α-L-arabinofuranosyl, feruloyl or p-coumaroyl, and so forth contribute to the formation of heteropolymeric structure. 29,31,32…”