A Source of Ultrasensitivity in the Glutamine Response of the Bicyclic Cascade System Controlling Glutamine Synthetase Adenylylation State and Activity in Escherichia coli

Glutamine synthetase (GS) activity in Escherichia coli is regulated by reversible adenylylation, brought about by a bicyclic system comprised of uridylyltransferase/uridylyl-removing enzyme (UTase/UR), its substrate, PIT, adenylyltransferase (ATase), and its substrate, GS. The modified and unmodified forms of PII produced by the upstream UTase/UR-PII cycle regulate the downstream ATase-GS cycle. A reconstituted UTase/UR-PII-ATase-GS bicyclic system has been shown to produce a highly ultrasensitive response of GS adenylylation state to the glutamine concentration, but its composite UTase/UR-PII and ATase-GS cycles displayed moderate glutamine sensitivities when examined separately. Glutamine sensitivity of the bicyclic system was significantly reduced when the trimeric PIT protein was replaced by a heterotrimeric form of PII that was functionally monomeric, and coupling between the two cycles was different in systems containing wild-type or heterotrimeric PII. Thus, the trimeric nature of PII played a role in the glutamine response of the bicyclic system. We therefore examined regulation of the individual AT (adenylylation) and AR (deadenylylation) activities of ATase by PIT preparations with various levels of uridylylation. AR activity was affected in a linear fashion by PIT uridylylation, but partially modified wild-type PII activated the AT much less than expected based on the extent of PII modification. Partially modified wild-type PIT also bound to ATase less than expected based upon the fraction of modified subunits. Our results suggest that the AT activity is only bound and activated by completely unmodified PII and that this design is largely responsible for ultrasensitivity of the bicyclic system.