A Novel Nitrate/Nitrite Permease in the Marine CyanobacteriumSynechococcus sp. Strain PCC 7002 release_ywaatxldmbbsvhcvxn2shvagma

Abstract

The <jats:italic>nrtP</jats:italic> and <jats:italic>narB</jats:italic> genes, encoding nitrate/nitrite permease and nitrate reductase, respectively, were isolated from the marine cyanobacterium <jats:italic>Synechococcus</jats:italic> sp. strain PCC 7002 and characterized. NrtP is a member of the major facilitator superfamily and is unrelated to the ATP-binding cassette-type nitrate transporters that previously have been described for freshwater strains of cyanobacteria. However, NrtP is similar to the NRT2-type nitrate transporters found in diverse organisms. An <jats:italic>nrtP</jats:italic> mutant strain consumes nitrate at a 4.5-fold-lower rate than the wild type, and this mutant grew exponentially on a medium containing 12 mM nitrate at a rate approximately 2-fold lower than that of the wild type. The <jats:italic>nrtP</jats:italic> mutant cells could not consume nitrite as rapidly as the wild type at pH 10, suggesting that NrtP also functions in nitrite uptake. A <jats:italic>narB</jats:italic> mutant was unable to grow on a medium containing nitrate as a nitrogen source, although this mutant could grow on media containing urea or nitrite with rates similar to those of the wild type. Exogenously added nitrite enhanced the in vivo activity of nitrite reductase in the <jats:italic>narB</jats:italic>mutant; this suggests that nitrite acts as a positive effector of nitrite reductase. Transcripts of the <jats:italic>nrtP</jats:italic> and<jats:italic>narB</jats:italic> genes were detected in cells grown on nitrate but were not detected in cells grown on urea or ammonia. Transcription of the<jats:italic>nrtP</jats:italic> and <jats:italic>narB</jats:italic> genes is probably controlled by the NtcA transcription factor for global nitrogen control. The discovery of a nitrate/nitrite permease in <jats:italic>Synechococcus</jats:italic>sp. strain PCC 7002 suggests that significant differences in nutrient transporters may occur in marine and freshwater cyanobacteria.
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