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Nicotinic acid in Rhodotorula glutinis is synthesized from tryptophan via N′-formyIkynurenine, kynurenine, 3-hydroxykynurenine and 3-hydroxyanthranilic acid. Auxotrophic mutants, with blocked synthesis at various steps between tryptophan and 3-hydroxyanthranilic acid, can grow with 100 μg/ml anthranilic acid (concentration in the agar-agar). The usually required concentration of substances such as nicotinic acid or other substances after the blocked step is about 2 μg/mI. Auxotrophs with a block between 3-hydroxyanthranilic acid and nicotinic acid are not able to grow with anthranilic acid. Therefore mutants requiring nicotinic acid with a blocked step between tryptophan and 3-hydroxyanthranilic acid can synthesize nicotinic acid from anthranilic acid, a substance formed in the first step of tryptophan synthesis from chorismic acid, by eluding the blocked pathways The same ability has been found in prototroph revertants with the original block of the auxotrophs but with a second mutation in the gene of anthranilate synthetase. As a result of this mutation the feedback inhibition by tryptophan is disturbed and anthranilic acid is produced in excess. When the culture medium of the prototrophs is analysed by paper chromatography after adding anthranilic acid to the growing culture, 3-hydroxyanthranilic acid as one of the formed products can be found. Summarizing these facts it is evident, that there exists a "short-cut"-pathway of nicotinic acid synthesis in Rh. glutinis, which is realized by direct hydroxylation of anthranilic acid to 3-hydroxyanthranilic acid. This "short-cut"pathway however is only possible, when anthranilic acid in the cell is present in excess. [ABSTRACT FROM AUTHOR]