Bacterial PEP-dependent carbohydrate: phosphotransferase systems couple sensing and global control mechanisms.

The PEP-dependent carbohydrate:phosphotransferase systems (PTSs) of enteric bacteria constitute a complex sensory system which involves as its central element a PEP-dependent His-protein kinase (Enzyme I). As a unit, the PTS comprises up to 20 different transporters per cell which correspond to its chemoreceptors for PTS carbohydrates, and several targeting subunits, which include in the low [G+C] Gram-positive bacteria an ancillary Ser/Thr-protein kinase. The PTS senses the presence of carbohydrates, in particular glucose, in the medium and the energy state of the cell, in the form of either the intracellular PEP-to-pyruvate ratio or the D-fructose-bisphosphate levels. This information is subsequently communicated to cellular targets, in particular those involved in the chemotactic response of the cell towards PTS carbohydrates, and in sensing glucose in the medium, using cAMP and several targeting subunits as intermediates. Peptide targeting subunits ensure the fast, transient, and yet accurate communication of the PTS with its more than hundred different targets, avoiding at the same time unwanted cross-talk. Many elements of this sensory system are simultaneously elements of specific and global regulatory networks. Thus, the PTS controls, besides the immediate (in the ms to s range) chemotactic responses, the activity of the various carbohydrate transporters and enzymes involved in carbon and energy metabolism through inducer exclusion, and in a delayed response (in the min to h range) the synthesis of these transporters and catabolic enzymes through catabolite repression. Indirect consequences of this program are phenomena related to cell surface rearrangements, which include flagella synthesis, as well as memory, adaptation, and learning effects. The analogy between the PTS and other prokaryotic systems, and more complex sensory systems from eukaryotic organisms which share elements with regulatory systems is obvious.