RESUMO
The genes lmbA,B1,B2 in the lincomycin A production gene cluster of Streptomyces lincolnensis were shown to form a common transcription unit with the promoter located directly upstream of lmbA. The proteins LmbB1 (mol. mass, 18 kDa) and LmbB2 (mol. mass 34 kDa), when over-produced together in Escherichia coli, brought about enzyme activities for the specific conversion of both L-tyrosine and L-3,4-dihydroxyphenylalanine (L-DOPA) to a yellow-colored product. The LmbB1 protein alone catalyzed the conversion of L-DOPA, but not of L-tyrosine. The purified LmbB1 protein showed a Km for L-DOPA of 258.3 microM. The L-tyrosine converting activity could not been demonstrated in vitro. The preliminary interpretation of these data suggests that the protein LmbB1 is an L-DOPA extradiol-cleaving 2,3-dioxygenase and that the protein LmbB2, either alone or in accord with LmbB1, represents an L-tyrosine 3-hydroxylase. This sequence of putative oxidation reactions on L-tyrosine seems to represent a new pathway different from the ones catalyzed by mammalian L-tyrosine hydroxylases or the wide-spread tyrosinases. The protein LmbA seemed not to be involved in this process. The labile, yellow-colored product from L-DOPA could not be converted to a picolinic acid derivative [3-(2-carboxy-5-pyridyl)alanine] in the presence of ammonia. Therefore, it probably is not a derivative of a cis, cis-3-hydroxymuconic acid semialdehyde; instead, its speculative structure represents a heterocyclic precursor of the propylhygric acid moiety of lincomycin A.
