A polyketide synthase ("PKS") of Type I is a complex multienzyme including
a loading domain linked to a multiplicity of extension domains. The first
extension module receives an acyl starter unit from the loading domain
and each extension module adds a further ketide unit which may undergo
processing (e.g. reduction). We have found that the Ksq domain possessed
by some PKS's has decarboxylating activity, e.g. generating (substituted)
acyl from (substituted) malonyl. The CLF domain of type II PKS's has
similar activity. By inserting loading modules including such domains
into PKS's not normally possessing them it is possible to control the
starter units used.
