We isolated from a tomato cDNA library the tomPRO1 locus, which encodes 
gamma-glutamyl kinase (GK) and gamma-glutamyl phosphate reductase (GPR). This 
locus is unusual among eukaryotic genetic elements because it contains two open 
reading frames, and thus resembles prokaryotic polycistronic operons. The first 
open reading frame, specifying GK, is terminated by a TAA codon, which is 
followed by five nucleotides, an ATG translation initiation codon, and the 
second open reading frame, encoding GPR. DNA sequence analysis of fragments 
obtained by PCR amplification confirmed that the internal TAA and neighboring 
sequences are present in the endogenous tomPRO1 sequence in tomato. We 
demonstrated with RNase protection assays that the tomPRO1 locus is transcribed 
in tomato tissue culture cells, into a product that contains the internal stop 
codon. In Escherichia coli, tomPRO1 directed the synthesis of two proteins, a 
33-kDa GK and a 44-kDa GPR. Antibodies against the 44-kDa GPR purified from E. 
coli recognized a 70-kDa product in tomato tissue culture cells and a 60-kDa 
product in leaves and roots. These results suggest that in tomato tissues, GPR 
is made as part of a longer polypeptide by some translational mechanism that 
enables bypass of the internal stop codon, such as frameshifting or ribosome 
hopping. The tomPRO1 locus may be the first example of a nuclear genetic element 
in plants that encodes two functional enzymes in two distinct open reading 
frames.