In most cyanobacteria, the only known pathway for oxidation of stored carbohydrate in the dark or under energy-limiting conditions is the hexose monophosphate shunt. To determine whether the increased use of the shunt under these conditions derives from an increase in the activity level of the respective enzymes, we measured the effect of growth phase during the growth of batch cultures of Synechococcus sp. strain PCC7942 on the specific activity of 6-phosphogluconate dehydrogenase (6PGD) and glucose 6-phosphate dehydrogenase. The specific activities were constant during the exponential growth phase of the culture, but they increased about fivefold during the transition into stationary phase. As an approach to determining the level of expression at which the growth-phase-dependent regulation of 6PGD level is exerted, we constructed operon and gene fusions between the gnd gene, which encodes 6PGD, and the Escherichia coli lacZ gene, which encodes beta-galactosidase (beta Gal). Strains harboring the fusions integrated into the cyanobacterial chromosome were prepared, and the growth-phase dependence of beta Gal level was determined. The specific activity of beta Gal in cultures of both types of fusion strains increased during the transition into stationary phase, indicating that the growth-phase-dependent regulation is on the gnd mRNA level. Characterization of the growth-phase-dependent induction of 6PGD in strains carrying differing amounts of DNA upstream from the gnd structural gene led to the localization of the promoter and the regulatory site on the restriction map of the gene, whose sequence has previously been determined.(ABSTRACT TRUNCATED AT 250 WORDS)