Fig. 2: NirP1 expression is mediated through a C_i-sensitive promoter and the transcription factor NtcA.

a Top: Native P_nirP1 promoter sequence from position −70 to +30 (TSS at +1) used in a bioluminescence reporter strain harboring a P_nirP1-luxAB transcriptional fusion. Functional promoter elements are colored blue. The previously determined²¹ transcription start site (TSS) is indicated. Middle row: Mutated nucleotides in the NtcA-binding site yielding promoter P_NtcA-Mut are colored red. Bottom: Mutated nucleotides in the repeat motif yielding promoter P_Repeat-Mut are colored red. Both mutated promoters were fused to luxAB and introduced into a neutral site in parallel with the P_nirP1-luxAB construct. b Bioluminescence of the Synechocystis 6803 P_nirP1-luxAB, P_NtcA-Mut-luxAB reporter, and P_Repeat-Mut-luxAB strains and a promoter-less (P_less) negative control. Cells were grown under HC conditions (BG11 supplemented with 10 mM NaHCO₃) for 2 h and transferred to BG11 medium without a CO₂ source to induce LC conditions for 24 h. c The strains were cultivated in BG11 (-N), and the chlorotic cultures were transferred back to standard BG11 (17.6 mM NaNO₃) medium to start the recovery process seven days after N starvation was initiated. d Strains were cultivated in standard BG11 and then 10 mM NH₄Cl was added for 24 h. Bioluminescence data are presented as the means ± SDs of 3 independent measurements with three biological replicates each (n = 9). Significance was calculated with a two-tailed t-test with unequal variance (Welch’s t-test; *P < 0.05; **P < 0.01; ***P < 0.001) between the strains at corresponding time points (details of statistical analysis Supplementary Fig. S3 and in Supplementary Dataset 3). Source data are provided as a Source Data file.