Cyanobacteria are rich in biologically active secondary metabolites, many of
which have potential application as anticancer or antimicrobial drugs or as useful
probes in cell biology studies. A Jamaican isolate of the marine cyanobacterium,
Lyngbya majuscula was the source of a novel antifungal and cytotoxic secondary
metabolite, hectochlorin. The structure of hectochlorin suggested that it was derived
from a hybid PKS/NRPS system. Unique features of hectochlorin such as the
presence of a gem dichloro functionality and two 2,3-dihydroxy isovaleric acid
prompted efforts to clone and characterize the gene cluster involved in hectochlorin
Initial attempts to isolate the hectochlorin biosynthetic gene cluster led to the
identification of a mixed PKS/NRPS gene cluster, LMcryl, whose genetic architecture
did not substantiate its involvement in the biosynthesis of hectochlorin. This gene
cluster was designated as a cryptic gene cluster because a corresponding metabolite
remains as yet unidentified. The expression of this gene cluster was successfully
demonstrated using RT-PCR and these results form the basis for characterizing the
metabolite using a novel interdisciplinary approach.
A 38 kb region putatively involved in the biosynthesis of hectochlorin has also
been isolated and characterized. The hct gene cluster consists of eight open reading
frames (ORFs) and appears to be colinear with regard to hectochlorin biosynthesis.
An unusual feature of this gene cluster includes the presence of a ketoreductase
domain in an NRPS module and appears to be the first report of such an occurrence in
a cyanobacterial secondary metabolite gene cluster. Other tailoring enzymes present
in the gene cluster are two cytochrome P450 monooxygenases and a putative
halogenase. The juxtaposition of two ORF's with identical modular organization
suggests that this gene cluster may have resulted from a gene duplication event.
Furthermore, biochemical characterization of two adenylation domains from this
cluster strengthens its involvement in the biosynthesis of hectochlorin. / Graduation date: 2006
|02 June 2005
|Ramaswamy, Aishwarya V.
|Gerwick, William H.
|Oregon State University
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