Part of the L-tyrosine degradation gene cluster that mediates the biosynthesis of the brownish pigment pyomelanin as an alternative melanin (PubMed:19028908, PubMed:22046314). The 4-hydroxyphenylpyruvate dioxygenase hppD catalyzes the conversion of 4-hydroxyphenylpyruvate to homogentisic acid (HGA) (PubMed:19028908, PubMed:22046314). The protein hmgX is crucial for this conversion and thus, probably functions as an accessory factor to mediate specific activity of hppD (PubMed:22046314). The homogentisate 1,2-dioxygenase hmgA is then involved in the cleavage of the aromatic ring of HGA and its conversion to 4-maleylacetoacetate (PubMed:19028908, PubMed:19715768). When hmgA activity is lowered by the cell wall integrity (CWI) signaling pathway, HGA accumulates and leads to the production of pyomelanin through benzoquinone acetic acid after oxidation and polymerization (PubMed:19715768). On the opposite, in non-stress conditions, both hppD and hmgA activities are balanced and HGA is degraded into 4-maleylacetoacetate (PubMed:19715768). 4-maleylacetoacetate is further converted to 4-fumarylacetoacetate by the maleylacetoacetate isomerase maiA, which is degraded into fumarate and acetoacetate by the fumarylacetoacetase fahA (Probable). Expression is induced by L-tyrosine (PubMed:22046314). Expression is positively regulated by the cluster-specific transcription factor hmgR (PubMed:22046314). Impairs growth on L-tyrosine as the sole carbon source and affects homogentisic acid and pyomelanin formation. Belongs to the TTC36 family.