The genes encoding the enzymes of IMP biosynthesis in constitute the operon, whereas the genes encoding GMP biosynthetic enzymes, (GMP synthetase) and (IMP dehydrogenase), and the gene encoding adenylosuccinate (sAMP) synthetase all occur as single units. The gene encodes an enzyme involved in both IMP and AMP biosynthesis and is located in the operon. The levels of purine biosynthetic enzymes (except for GMP synthetase) were repressed in cells grown in the presence of purine compounds. Transcription of the operon is regulated negatively by adenine and guanine compounds. Our results suggest that ATP and guanine (or hypoxanthine) act as low molecular mass repressors. The level of IMP dehydrogenase was repressed by guanosine, but not in the presence of adenine, and was negatively correlated with the GTP/ATP pools ratio. The level of sAMP synthetase was repressed by adenine and increased by guanosine, and was positively correlated with the GTP/ATP pools ratio. It appears that the mode of regulating purine biosynthetic enzyme levels coincides with the cellular need for the individual enzymes.


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