Escherichia coli uses three kinds of phosphorus compounds for growth: inorganic phosphate (Pi), organophosphates, and phosphonates (Pn).
When Pi, the preferred phosphorus (PI source, is in excess, Pi is taken up by the low affinity Pi transporter, Pit. Under these conditions, the genes for the high affinity Pi-specific transporter, Pst, and ones for use of alternative P sources are repressed. The latter genes are coregulated as members of the phosphate (PHO) regulon and are induced more than 100-fold during Pi limitation. Altogether 31 genes belonging t o the E. coli PHO regulon have now been cloned and sequenced. Most of the corresponding gene products have also been characterized. They are transcribed as eight separate genes and operons. All probably have a role in the assimilation of different P sources from the environment (Table I).
Cellular P metabolism is complex. Hence, it is likely that multiple controls may act on the PHO regulon. Basically, the assimilation of any P compound involves two early steps. First, Pi or an alternative P compound must be taken up. And, second, the Pi, or the P in the alternative compound must be incorporated into ATP, the primary phosphoryl donor in metabolism. Eventually, P is incorporated into essential compo-