Transcriptional analysis of the queD gene coding for quercetinase of Streptomyces sp. FLA.

Quercetinase, catalyzing the 2,4-dioxygenolytic cleavage of the flavonol quercetin (3,5,7,3',4'-pentahydroxyflavone) to carbon monoxide and 2-protocatechuoylphloroglucinol carboxylic acid, is encoded by the queD gene in Streptomyces sp. FLA. Because studies on the transcriptional regulation of quercetinase genes are rare, we analyzed the expression of queD in response to quercetin and other carbon compounds. RNA hybridization experiments revealed that transcription of queD is triggered by quercetin and its 3-O-rhamnosylglucoside rutin, but not by the flavonol morin (3,5,7,2',4'-pentahydroxyflavone), the presumed quercetin degradation products protocatechuate and 2,4,6-trihydroxybenzoate or the sugars rhamnose and glucose. Quercetin-induced queD expression was not influenced by the presence of Ni(II), the preferred cofactor of Streptomyces QueD. Reverse transcription-PCR analysis showed a concerted transcription of queD and two putative genes located downstream of queD, which were predicted to code for an amidohydrolase and an esterase. By determination of the transcriptional start site of the queD operon, putative -10 and -35 regions could be identified, suggesting transcription from a sigma70-dependent promoter. Sequence analysis of the queD promoter region indicated possible binding sites for an LmrA/YxaF-like repressor.