Transcription factor NtcB specifically controls the nitrate assimilation genes in the marine cyanobacterium Synechococcus sp strain PCC 7002

The ntcA and ntcB genes, encoding cAMP-receptor-protein (CRP) type and LysR type transcriptional regulators, respectively, were isolated from the transformable marine cyanobacterium Synechococcus sp. strain PCC 7002 to study the inactivation of these genes by interposon mutagenesis. The ntcA gene could not be disrupted in this cyanobacterial strain, indicating that the ntcA gene is essential for cell growth on either urea or nitrate as the sole nitrogen source. The ntcB mutant cells grew very slowly on nitrate or nitrite as the sole nitrogen source but could grow on urea at the same rate as wild-type cells. The consumption of nitrate and nitrite was barely detectable in the ntcB mutant cells, indicating that nitrate and nitrite use was impaired in the ntcB mutant. In wild-type cells, mRNA levels for the three genes required for nitrate assimilation, nrtP (nitrate permease), narB (nitrate reductase) and nirA (nitrite reductase), increased when cells were shifted to nitrate growth conditions, but no increase in transcription of these genes occurred in the ntcB mutant. The mRNA level of ntcA was nearly identical in the wild-type and ntcB mutant cells. These results demonstrate that NtcB specifically controls the transcription of the three genes for nitrate assimilation in this marine cyanobacterium.