CIRCADIAN EXPRESSION OF GENES INVOLVED IN THE PURINE BIOSYNTHETIC-PATHWAY OF THE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC-7942

Extensive circadian (daily) control over gene expression in the cyanob acterium Synechococcus sp. strain PCC 7942 is programmed into at least two differentially phased groups. The transcriptional activity of the smaller group of genes is maximal at about dawn and minimal at about dusk. We identified one of the genes belonging to this latter group as purF, which encodes the key regulatory enzyme in the de novo purine s ynthetic pathway, glutamine PRPP amido-transferase (also known as amid ophosphoribosyltransferase). its expression pattern as a function of c ircadian time was confirmed by both luminescence from a purF::luxAB re porter strain and the abundance of purF mRNA. By fusing sequences upst ream of the purF coding region to promoterless luxAB genes, we identif ied a limited upstream region, which potentially regulates purF circad ian expression patterns in vivo. We also identified the purl gene imme diately upstream of purF. The purl gene encodes FGAM synthetase, the f ourth enzyme in the purine nucleotide biosynthesis pathway. Although t hese genes are expressed as part of a larger operon in other bacteria, reporter gene fusions revealed that purF and purl are transcribed ind ependently in Synechococcus and that they are expressed at different p hases of the circadian cycle. This differential expression pattern may be related to the oxygen sensitivity of amidophosphoribosyltransferas e.