We studied the effect of molybdenum (Mo) concentration on transcription and translation of a putative Mo-storage protein (Mop) in the freshwater heterocystous cyanobacterium, Nostoc sp. PCC 7120. Triplicate treatments were acclimated to 1, 150, and 3000 nM Mo over an 11-day period (three transfers) and then transferred into 3000 nM Mo media. Growth rates in 1 nM treatments declined throughout the acclimation period and increased significantly after the final transfer into media containing 3000 nM Mo. After acclimation, cellular Mo content was highest in 3000 nM Mo treatments, intermediate in 150 nM treatments and lowest in 1 nM treatments (70 ± 30, 10.0 ± 0.04 and 2 ± 1 mg·gˉ1 dry biomass, respectively). Cellular Mo content converged on values of 20-40 mg·gˉ1 dry biomass after the final transfer into 3000 nM. Mop transcription and translation were up-regulated in 1 nM Mo treatments during the acclimation period, and down-regulated after transfer into 3000 nM Mo. Mop protein expression was only observed in 1 nM treatments after multiple transfers; minimal Mop protein was observed in 150 and 3000 nM Mo treatments. These observations suggest that Mop does not store excess intracellular Mo in Nostoc sp. PCC 7120, but may serve an unknown physiological function in Mo-limited metabolism.
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