Figure 6.

Schematic representation of the role of PNPase in poly(A) tail metabolism in wild-type E. coli. After primary polyadenylylation of the transcript by PAP I, PNPase may bind to the 3′ end of the poly(A) tail. Depending on the availability of inorganic phosphate (Pi), PNPase works either degradatively or biosynthetically. In the presence of high Pi concentration, it degrades the poly(A) tail releasing diphosphates. If the Pi concentration is low, it works biosynthetically adding one or more nucleotides to the existing poly(A) tail and in the process generates inorganic phosphate. On dissociation, the 3′ end again is available to PAP I for further polymerization.