Fig. 1.

The reductive pathway for the degradation of pyrimidine nucleotides in Arabidopsis. Names of enzymes catalysing each reaction are given with the AGI locus and gene name. The asterisk indicates C-2 of the pyrimidine ring, which is released as CO₂ in the PYD3 reaction. The N-3 pyrimidine nitrogen (light highlighting) is also released as ammonia at this step. The fourth step, in which the putative β-alanine aminotransferase (BAT) transfers pyrimidine N-1 (darker highlighting) to pyruvate or 2-oxoglutarate to form l-alanine or l-glutamate, respectively, is not considered to be part of the reductive pathway. It is included here to illustrate the route by which both pyrimidine nitrogen atoms are assimilated into general nitrogen metabolism.