Ammonia is almost invariably the preferred source of nitrogen
for bacterial growth in that it supports a higher growth rate
than any other nitrogen source. However, bacteria frequently
have to utilize a wide range of alternative nitrogen sources, and
to accomplish this they are capable of synthesizing numerous
proteins for the uptake and subsequent metabolism of nitrogenous
compounds. The synthesis, and in some cases the activity,
of these proteins is tightly regulated in concert with the
availability of their substrates, and this regulation, which was
first recognized in the pioneering work of Magasanik and coworkers
on histidase synthesis in Klebsiella aerogenes (199,
200), is now termed nitrogen control. The biochemical and
genetic mechanisms that facilitate nitrogen control are undoubtedly
most clearly understood in the enteric bacteria, although
elements of the system are now recognized in many
other bacterial groups.