An N-terminal
(His)6-PHA synthase was constructed and puri®ed
with speci®c activity of 40 U/mg. The variable activity
is shown to be related to the protein's propensity to
aggregate and not to incomplete post-translational
modi®cation by coenzyme A and a phosphopantetheinyl
transferase. The substrate speci®cities of this enzyme
and the Class III PHA synthase from Allochromatium
vinosum have been determined with nine analogs
of varied chain length and branching, OH group
position within the chain, and thioesters. The results
suggest that in vitro, both PHA synthases are very
speci®c and provide further support for their active
site structural similarities. In vitro results differ from
studies in vivo.