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Abstract
Quality traits of raspberry fruits such as aroma and color derive in part from the
polyketide derivatives, benzalacetone and dihydrochalcone, respectively. The formation
of these metabolites during fruit ripening is the result of the activity of polyketide
synthases (PKS), benzalcetone synthase and chalcone synthase (CHS), during fruit development.
To gain an understanding of the regulation of these multiple PKSs during fruit ripening,
we have characterized the repertoire of Rubus PKS genes and studied their expression
patterns during fruit ripening. Using a PCR-based homology search, a family of ten
PKS genes (Ripks1-10) sharing 82-98% nucleotide sequence identity was identified in
the Rubus idaeus genome. Low stringency screening of a ripening fruit-specific cDNA
library, identified three groups of PKS cDNAs. Group 1 and 2 cDNAs were also represented
in the PCR amplified products, while group 3 represented a new class of Rubus PKS
gene. The Rubus PKS gene-family thus consists of at least eleven members. The three
cDNAs exhibit distinct tissue-specific and developmentally regulated patterns of expression.
RiPKS5 has high constitutive levels of expression in all organs, including developing
flowers and fruits, while RiPKS6 and RiPKS11 expression is consistent with developmental
and tissue-specific regulation in various organs. The recombinant proteins encoded
by the three RiPKS cDNAs showed a typical CHS-type PKS activity. While phylogenetic
analysis placed the three Rubus PKSs in one cluster, suggesting a recent duplication
event, their distinct expression patterns suggest that their regulation, and thus
function(s), has evolved independently of the structural genes themselves.