Genes and enzymes involved in caffeic acid biosynthesis in the actinomycete Saccharothrix espanaensis

The saccharomicins A and B, produced by the actinomycete Saccharothrix espanaensis, are oligosaccharide antibiotics. They consist of 17 monosaccharide units and the unique aglycon N-(m,p-dihydroxycinnamoyl)taurine. To investigate candidate genes responsible for the formation of trans-m,p-dihydroxycinnamic acid (caffeic acid) as part of the saccharomicin aglycon, gene expression experiments were carried out in Streptomyces fradiae XKS. It is shown that the biosynthetic pathway for trans-caffeic acid proceeds from L-tyrosine via trans-p-coumaric acid directly to trans-caffeic acid, since heterologous expression of sam8, encoding a tyrosine ammonia-lyase, led to the production of trans-p-hydroxycinnamic acid (coumaric acid), and coexpression of sam8 and sam5, the latter encoding a 4-coumarate 3-hydroxylase, led to the production of trans-m,p-dihydroxycinnamic acid. This is not in accordance with the general phenylpropanoid pathway in plants, where trans-p-coumaric acid is first activated before the 3-hydroxylation of its ring takes place.