Structure/function analysis of a type iii polyketide synthase in the brown alga Ectocarpus siliculosus reveals a biochemical pathway in phlorotannin monomer biosynthesis

Plant Cell. 2013 Aug;25(8):3089-103. doi: 10.1105/tpc.113.111336. Epub 2013 Aug 27.


Brown algal phlorotannins are structural analogs of condensed tannins in terrestrial plants and, like plant phenols, they have numerous biological functions. Despite their importance in brown algae, phlorotannin biosynthetic pathways have been poorly characterized at the molecular level. We found that a predicted type III polyketide synthase in the genome of the brown alga Ectocarpus siliculosus, PKS1, catalyzes a major step in the biosynthetic pathway of phlorotannins (i.e., the synthesis of phloroglucinol monomers from malonyl-CoA). The crystal structure of PKS1 at 2.85-Å resolution provided a good quality electron density map showing a modified Cys residue, likely connected to a long chain acyl group. An additional pocket not found in other known type III PKSs contains a reaction product that might correspond to a phloroglucinol precursor. In vivo, we also found a positive correlation between the phloroglucinol content and the PKS III gene expression level in cells of a strain of Ectocarpus adapted to freshwater during its reacclimation to seawater. The evolution of the type III PKS gene family in Stramenopiles suggests a lateral gene transfer event from an actinobacterium.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acclimatization
  • Amino Acid Sequence
  • Biosynthetic Pathways*
  • Catalytic Domain
  • Crystallography, X-Ray
  • Fresh Water
  • Gene Expression Regulation, Enzymologic
  • Gene Expression Regulation, Plant
  • Models, Molecular
  • Molecular Sequence Data
  • Phaeophyta / enzymology*
  • Phaeophyta / genetics
  • Phloroglucinol / chemistry
  • Phloroglucinol / metabolism
  • Phylogeny
  • Polyketide Synthases / chemistry*
  • Polyketide Synthases / genetics
  • Polyketide Synthases / metabolism*
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • Recombinant Proteins / metabolism
  • Seawater
  • Sequence Alignment
  • Structure-Activity Relationship
  • Tannins / biosynthesis*
  • Tannins / chemistry


  • RNA, Messenger
  • Recombinant Proteins
  • Tannins
  • Polyketide Synthases
  • Phloroglucinol

Associated data

  • GENBANK/CBJ28635
  • GENBANK/CBJ48712
  • GENBANK/CBN76919
  • PDB/4B0N