One amino acid makes the difference: the formation of ent-kaurene and 16α-hydroxy-ent-kaurane by diterpene synthases in poplar.
BACKGROUND
Labdane-related diterpenoids form the largest group among the diterpenes. They fulfill important functions in primary metabolism as essential plant growth hormones and are known to function in secondary metabolism as, for example, phytoalexins. The biosynthesis of labdane-related diterpenes is mediated by the action of class II and class I diterpene synthases. Although terpene synthases have been well investigated in poplar, little is known about diterpene formation in this woody perennial plant species.


RESULTS
The recently sequenced genome of Populus trichocarpa possesses two putative copalyl diphosphate synthase genes (CPS, class II) and two putative kaurene synthase genes (KS, class I), which most likely arose through a genome duplication and a recent tandem gene duplication, respectively. We showed that the CPS-like gene PtTPS17 encodes an ent-copalyl diphosphate synthase (ent-CPS), while the protein encoded by the putative CPS gene PtTPS18 showed no enzymatic activity. The putative kaurene synthases PtTPS19 and PtTPS20 both accepted ent-copalyl diphosphate (ent-CPP) as substrate. However, despite their high sequence similarity, they produced different diterpene products. While PtTPS19 formed exclusively ent-kaurene, PtTPS20 generated mainly the diterpene alcohol, 16α-hydroxy-ent-kaurane. Using homology-based structure modeling and site-directed mutagenesis, we demonstrated that one amino acid residue determines the different product specificity of PtTPS19 and PtTPS20. A reciprocal exchange of methionine 607 and threonine 607 in the active sites of PtTPS19 and PtTPS20, respectively, led to a complete interconversion of the enzyme product profiles. Gene expression analysis revealed that the diterpene synthase genes characterized showed organ-specific expression with the highest abundance of PtTPS17 and PtTPS20 transcripts in poplar roots.


CONCLUSIONS
The poplar diterpene synthases PtTPS17, PtTPS19, and PtTPS20 contribute to the production of ent-kaurene and 16α-hydroxy-ent-kaurane in poplar. While ent-kaurene most likely serves as the universal precursor for gibberellins, the function of 16α-hydroxy-ent-kaurane in poplar is not known yet. However, the high expression levels of PtTPS20 and PtTPS17 in poplar roots may indicate an important function of 16α-hydroxy-ent-kaurane in secondary metabolism in this plant organ.
Max Planck Institute for Chemical Ecology, Hans-Knöll-Strasse 8, D-07745, Jena, Germany. sirmisch@ice.mpg.de.
Sandra
S
Irmisch
Max Planck Institute for Chemical Ecology, Hans-Knöll-Strasse 8, D-07745, Jena, Germany. amueller@ice.mpg.de.
Andrea T
AT
Müller
Max Planck Institute for Chemical Ecology, Hans-Knöll-Strasse 8, D-07745, Jena, Germany. lschmidt@ice.mpg.de.
Lydia
L
Schmidt
Max Planck Institute for Chemical Ecology, Hans-Knöll-Strasse 8, D-07745, Jena, Germany. jguenther@ice.mpg.de.
Jan
J
Günther
Max Planck Institute for Chemical Ecology, Hans-Knöll-Strasse 8, D-07745, Jena, Germany. gershenzon@ice.mpg.de.
Jonathan
J
Gershenzon
Max Planck Institute for Chemical Ecology, Hans-Knöll-Strasse 8, D-07745, Jena, Germany. koellner@ice.mpg.de.
Tobias G
TG
Köllner
D019883:Alkyl and Aryl Transferases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D000596:Amino Acids / Q000378:metabolism*; D045786:Diterpenes, Kaurane / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D017343:Genes, Plant; D008274:Magnesium / Q000494:pharmacology; D008969:Molecular Sequence Data; D010802:Phylogeny; D032107:Populus / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity / Q000187:drug effects
D000596:Amino Acids; D045786:Diterpenes, Kaurane; D012333:RNA, Messenger; D019883:Alkyl and Aryl Transferases; C108877:terpene synthase; D008274:Magnesium