Seryl-tryptophan

The adverse effects of drought on plants are gradually exacerbated with global climatic change. Amelioration of the drought stress that is induced by low doses of acetic acid (AA) has been caused great interest in plants. However, whether AA can change soil microbial composition is still unknown. Here, we investigated how exogenous AA regulates the physiology, rhizosphere soil microorganisms and metabolic composition on Salix myrtillacea under drought stress. The physiological results showed that AA could improve the drought tolerance of S. myrtillacea. Azotobacter and Pseudomonas were enriched in the rhizosphere by AA irrigation. AA significantly increased the relative contents of amino acid metabolites (e.g., glycyl-L-tyrosine, l-glutamine and seryl-tryptophan) and decreased the relative contents of phenylpropane metabolites (e.g., fraxetin and sinapyl aldehyde) in soils. The enrichments of Azotobacter and Pseudomonas were significantly correlated with glycyl-L-tyrosine, l-glutamine, seryl-tryptophan, fraxetin and sinapyl aldehyde, which could increase the stress resistance by promoting nitrogen (N) uptake for willows. Furthermore, inoculation with Azotobacter chroococcum and Pseudomonas fluorescens could significantly improve willows drought tolerance. Therefore, our results reveal that the changes of plant physiology, rhizosphere soil microorganisms and metabolic composition induced by AA can improve willows drought resistance by enhancing N uptake.

Two zinc-metalloendopeptidases, GFMEP (accession number P81054) and POMEP (accession number P81055), from the fruiting bodies of two edible mushrooms, Grifola frondosa and Pleurotus ostreatus, respectively, specifically hydrolyze peptidyl-lysine bonds (-X-Lys-) in polypeptides. To understand detailed substrate specificities and kinetic characters of these enzymes, we have synthesized various intramolecularly quenched fluorescent peptide substrates and determined their kinetic constants with these substrates. Each synthesized fluorogenic peptide has a fluorescent residue, tryptophan, at its carboxyl terminus and a quenching group, dinitrophenyl (Dnp), at its amino terminus. Quenching of the Trp fluorescence in an intact substrate is relieved on hydrolysis of the -X-Lys- bond, giving rise to a continuous increase in fluorescence. The octapeptide substrate, Dnp-Ser-Thr-Ala-Thr-Lys-Leu-Ser-Trp, was an efficient substrate for both enzymes, the kcat/Km values being 9.8 x 10(6) and 7.0 x 10(5) M-1.s-1 for GF- and POMEP, respectively. Peptides with aspartic acid adjacent to the Lys residue were found to be poor substrates for both enzymes. Neither the shortest peptide, Dnp-Thr-Lys-Trp, nor peptides with substitution of L-Arg, L-ornithine, or D-Lys for Lys were hydrolyzed by either enzyme. These results confirmed the strict specificities of GF- and POMEP toward the peptide bond, -X-Lys-. Substitution of Co2+ for Zn2+ enhanced the activity, while the Km values were comparable. All peptides not hydrolyzed by either enzyme had inhibitory effects on GFMEP activity. The active site structure is discussed in relation to these observations.