The Syringate -Demethylase Gene of sp. Strain SYK-6 Is Regulated by DesX, while Other Vanillate and Syringate Catabolism Genes Are Regulated by DesR.

Syringate and vanillate are the major metabolites of lignin biodegradation. In sp. strain SYK-6, syringate is O demethylated to gallate by consecutive reactions catalyzed by DesA and LigM, and vanillate is O demethylated to protocatechuate by a reaction catalyzed by LigM. The gallate ring is cleaved by DesB, and protocatechuate is catabolized via the protocatechuate 4,5-cleavage pathway. The transcriptions of , , and are induced by syringate and vanillate, while those of and are negatively regulated by the MarR-type transcriptional regulator DesR, which is not involved in regulation. Here, we clarified the regulatory system for transcription by analyzing the IclR-type transcriptional regulator , located downstream of Quantitative reverse transcription (RT)-PCR analyses of a mutant indicated that the transcription of was negatively regulated by DesX. In contrast, DesX was not involved in the regulation of and The ferulate catabolism genes (), under the control of a MarR-type transcriptional regulator, FerC, are located upstream of RT-PCR analyses suggested that the -SLG_25010- gene cluster consists of the operon and the SLG_25010- operon. Promoter assays revealed that a syringate- and vanillate-inducible promoter is located upstream of SLG_25010. Purified DesX bound to this promoter region, which overlaps an 18-bp inverted-repeat sequence that appears to be essential for the DNA binding of DesX. Syringate and vanillate inhibited the DNA binding of DesX, indicating that the compounds are effector molecules of DesX. Syringate is a major degradation product in the microbial and chemical degradation of syringyl lignin. Along with other low-molecular-weight aromatic compounds, syringate is produced by chemical lignin depolymerization. Converting this mixture into value-added chemicals using bacterial metabolism (i.e., biological funneling) is a promising option for lignin valorization. To construct an efficient microbial lignin conversion system, it is necessary to identify and characterize the genes involved in the uptake and catabolism of lignin-derived aromatic compounds and to elucidate their transcriptional regulation. In this study, we found that the transcription of , encoding syringate -demethylase in SYK-6, is regulated by an IclR-type transcriptional regulator, DesX. The findings of this study, combined with our previous results on (encoding a MarR transcriptional regulator that controls the transcription of and ), provide an overall picture of the transcriptional-regulatory systems for syringate and vanillate catabolism in SYK-6.