5' structure and expression of human glucose-6-phosphate dehydrogenase mRNA

The human glucose-6-phosphate (G6PD) cDNAs cloned from normal and carcinoma cells can encode 545-amino-acid residues starting from the first in-frame chain initiation codon. However, it was reported that the G6PD mRNAs of carcinoma cell lines were shorter and could encode only 515-amino-acid residues (Martini et al., 1986). We demonstrated the existence of two major G6PD mRNAs in normal reticulocytes, lymphoblasts, and hepatocytes by the primer extension analysis. The longer mRNA has a cap site at approx. nucleotide -166 and can encode 545-amino-acid residues, whereas the shorter mRNA has a cap site at approx. nucleotide -66, and encodes 515-amino-acid residues. These two naturally existing mRNAs (cDNAs) and an artificially truncated mRNA, which can encode the carboxy-terminal 479-amino-acid residues of the subunit, were expressed in the in vitro reticulocyte and wheat germ systems and in the in vivo E. coli system. All three species of mRNA (cDNA) were efficiently translated and produced proteins with the expected molecular sizes. The peptide with 515 residues formed the catalytically active enzyme, but the 545-residue protein and the 479-residue protein were catalytically inactive. The larger 545-residue protein may correspond to the larger G6PD precursor observed in the rat. The extended amino-terminal region encoded by the larger mRNA contains the -Arg-Gly-Gly-Arg-Arg-Arg-Arg-sequence, which is conserved in the nucleotide-binding protamine family.