Mannitol, a compatible solute synthesized by Acinetobacter baylyi in a two‐step pathway including a salt‐induced and salt‐dependent mannitol‐1‐phosphate dehydrogenase

Abstract

The nutritionally versatile and naturally competent soil bacterium Acinetobacter baylyi copes with salt stress by the accumulation of compatible solutes. NMR analyses revealed that cells amassed glutamate and the rather unusual sugar alcohol mannitol upon an increase of the external NaCl concentration. To unravel the path of mannitol biosynthesis, the genome was inspected for genes potentially involved in its biosynthesis. A gene encoding a potential mannitol-1-phosphate dehydrogenase (mtlD) was identified in the genome of A. baylyi. Expression of mtlD was highly induced at high salinity. mtlD was overexpressed and the purified protein indeed produced mannitol-1-phosphate from fructose-6-phosphate. The enzyme preferred NADPH over NADH and the specific activity of fructose-6-phosphate reduction with NADPH was 130 U mg(-1) . Enzymatic activity was strictly salt-dependent. Deletion of mtlD resulted in a complete loss of salt-dependent mannitol biosynthesis. We provide clear evidence that osmo-induced synthesis of the compatible solute mannitol is by a two-step pathway and that the mannitol-1-phosphate dehydrogenase mediating the first step of this pathway is regulated by salinity on the transcriptional as well as on the activity level.

Salt induction and activation of MtlD, the key enzyme in the synthesis of the compatible solute mannitol in Acinetobacter baumannii

“…Acinetobacter species cope with low water activities by the accumulation of compatible solutes. The nonpathogen A. baylyi synthesizes glutamate and mannitol de novo in response to increasing osmolarities of the medium (Sand, Mingote, Santos, Müller, & Averhoff, ). If glycine betaine or choline are present they are taken up from the environment (Sand et al., ).…”

“…It serves as carbon and energy source or radical scavenger and in plants and fungi it is well known as compatible solute (Chaturvedi, Wong, & Newman, ; Stoop, Williamson, & Pharr, ). In bacteria, only Pseudomonas putida (Kets, Galinski, de Wit, de Bont, & Heipieper, ), A. baylyi (Sand et al., ), and Gluconobacter oxydans (Zahid, Schweiger, Galinski, & Deppenmeier, ) have been reported to accumulate the compatible solute mannitol. The biosynthetic route for the production of mannitol as compatible solute in A. baylyi was unraveled only recently.…”

Molecular and biochemical characterization of mannitol-1-phosphate dehydrogenase from the model brown alga Ectocarpus sp.

“…These latter sequences represent short chain M1PDHs, compared to their algal and apicomplexa counterparts which are long chain M1PDHs. Interestingly, other types of proteins catalyzing M1PDH activity have been demonstrated in the soil bacterium Acinetobacter (Sand et al, 2013(Sand et al, , 2014, and the mammal pathogen Cr. neoformans (Suvarna et al, 2000).…”

“…All M1PDH enzymes have a high preference for using NAD(H) instead of NADP(H), except the M1PDH of Acenitobacter sp. which prefers NADP(H) (Sand et al, 2013), and the E. tenella enzyme which could utilize equally both NADP(H) and NAD(H) (Schmatz et al, 1989). Ectocarpus sp.…”

The Mannitol Utilization System of the Marine Bacterium Zobellia galactanivorans

“…In these terrestrial bacteria, mannitol is taken up by a mannitol-specific phosphoenolpyruvate/carbohydrate phosphotransferase system (PTS) and is phosphorylated into M1P during its transport, and M1P is further oxidized to F6P by a M1P-specific dehydrogenase (25) before entering glycolysis. In the soil bacterium Acinetobacter baylyi (26), the M1P dehydrogenase is fused to a haloacid dehalogenase (HAD)-like phosphatase domain at the N terminus that was shown to catalyze M1Pase activity (27). In C. acetobutylicum (23) and B. stearothermophilus (22), the mannitol catabolic operon is regulated by two mechanisms: a glucose-mediated catabolite repression and a transcriptional activation mechanism controlled by MtlR using mannitol as an inducer.…”