Liamocin biosynthesis is induced by an autogenous host acid activation in Aureobasidium melanogenum

Abstract

It has been known that maximal liamocin production must be carried out at low environmental pH (around 3.0). In this study, it was found that the low pH was mainly caused by the secreted citric acid which is one precursor of acetyl‐CoA for liamocin biosynthesis. Determination of citric acid in the culture, deletion, complementation and overexpression of the CEXA gene encoding specific citrate exporter demonstrated that the low pH was indeed caused by the secreted citric acid. Deletion, complementation and overexpression of the ACL gene encoding ATP‐citric acid lyase and effects of different initial pHs and added citric acid showed that the low pH in the presence of citric acid were suitable for lysis of intracellular citric acid, liamocin production and expression of the PACC gene encoding the pH signaling transcription factor PacC. This meant that the PACC gene was an acid‐expression gene. Deletion, complementation and overexpression of the PACC gene indicated that expression of the key gene cluster GAL1‐EST1‐PKS1 for liamocin biosynthesis was driven by the pH signaling transcription factor PacC and there was weak nitrogen catabolite repression on liamocin biosynthesis at the low pH. That was why liamocin biosynthesis was induced at the low pH in the presence of citric acid. The mechanisms of the enhanced liamocin biosynthesis by the autogenous host acid activation, together with the pH signaling pathway, were proposed.This article is protected by copyright. All rights reserved

Liamocins overproduction via the two‐pH stage fermentation and anti‐Aspergillus flavus activity of Massoia lactone

“…Under such acid conditions (pH 3.0), the autogenous host acid activates the transcriptional activator PacC with three‐zinc fingers (Cys 2 His 2 ) in the pH signaling pathway, indicating that the PACC gene encoding the transcriptional factor in the pH signaling pathway was an acid‐expressed gene in A. melanogenum . [ 15 ] We also found that the low pH (3.0) adjusted with HCl has positive influence on liamocin production. [ 15 ] It was discovered that overexpression the PK gene for phosphoketolase, the PDC gene encoding pyruvate decarboxylase and VHb gene coding for Vitreoscilla hemoglobin (VHb) in the glucose de‐repression mutants can make the engineered strain V33 produce 55.38 g L −1 of liamocin within 168 h during the 10‐L fermentation [ 17 ] while its wild type strain 9‐1 only produce 30.61 g L −1 of liamocin from 117.60 g L −1 of glucose within 168 h of the same fermentation.…”

“…[ 15 ] We also found that the low pH (3.0) adjusted with HCl has positive influence on liamocin production. [ 15 ] It was discovered that overexpression the PK gene for phosphoketolase, the PDC gene encoding pyruvate decarboxylase and VHb gene coding for Vitreoscilla hemoglobin (VHb) in the glucose de‐repression mutants can make the engineered strain V33 produce 55.38 g L −1 of liamocin within 168 h during the 10‐L fermentation [ 17 ] while its wild type strain 9‐1 only produce 30.61 g L −1 of liamocin from 117.60 g L −1 of glucose within 168 h of the same fermentation. [ 17 ]…”

“…In this study, it was also discovered that the constant pH 3.0 promoted expression of all the relevant genes for liamocin biosynthesis, leading to the highest amount of produced liamocin. In another study, [ 15 ] it was also shown that the native pH 3.0 caused by secreted citric acid in the same 9‐1 strain and the initial pH 3.0 of the medium were the most suitable for liamocin production and expression of the relevant genes for liamocin biosynthesis, regulation, transport, and secretion. In our previous studies, [ 3 ] it has been confirmed that the HR‐Pks encoded by the PKS1 gene is responsible for synthesis of 3,5‐dihydroxydecanoic acid tail in liamocin molecule and the Est1 encoded by the EST1 gene catalyzes ester bond formation between 3,5‐dihydroxydecanoic acid and mannitol or arabitol (heads) in liamocin molecule.…”

“…It has been evidenced that at natural pH 3.0 caused by the produced citric acid, the same engineered strain V33 yielded 55.38 g L −1 of liamocin. [ 15 ] This meant that the constant pH 3.0 adjusted with 2.0 M HCl was more suitable for liamocin biosynthesis than the natural pH 3.0 caused by secreted citric acid.…”

“…[1] It is interesting to have observed that the optimal liamocin production must be carried out at low pH (around 3.0). [13,15,16] Very recently, it has been strongly confirmed that the low pH is caused by the secreted citric acid from the producer via citric acid transporter CexA. Deletion of the CEXA gene encoding the citric acid transporter CexA made the Δcexa mutant lose the ability to secrete any citric acid, cause pH increases in the culture, reduce biosynthesis of liamocins and down-regulate expression of the acid-expressed PACC gene encoding the transcriptional activator PacC and any other genes for liamocin biosynthesis and regulation.…”