The tyrosine degradation gene hppD is transcriptionally activated by HpdA and repressed by HpdR in Streptomyces coelicolor, while hpdA is negatively autoregulated and repressed by HpdR.

Streptomyces coelicolor produces a brown pigment on nutrient-limited agar medium (Tyr-PM) using l-tyrosine as the sole nitrogen and carbon source. The pigment production is associated with the second step of l-tyrosine catabolism catalysed by 4-hydroxyphenylpyruvate dioxygenase (HppD), which converts 4-hydroxyphenylpyruvate (4HPP) to 2, 5-dihydroxyphenylacetate (homogentisate) to provide the carbon and energy substrates for the growth of S. coelicolor on Tyr-PM. An hppD mutant did not produce brown pigment, and its normal growth was impaired on Tyr-PM. hpdA and hpdR, located close to hppD, were identified as activator and repressor genes for hppD transcription in the presence of tyrosine. hpdA, divergently transcribed from hppD, is negatively autoregulated in the absence of tyrosine, whereas it is repressed by both its own protein and HpdR in the presence of tyrosine. Electrophoretic mobility shift assays and footprinting experiments showed that HpdA and HpdR each bind to an overlapping region spanning the promoters of both hppD and hpdA, and that 4HPP, instead of tyrosine, is the specific ligand modulating the binding patterns and footprints of HpdA and HpdR on the hppD-hpdA promoter region. These results suggested that the transcription of hppD is subject to coarse and fine control by a complex regulatory system.