Abstract
Transfer of ammonium-grown cells of Synechococcus elongatus to nitrogen-free medium increased expression levels of -80 genes involved in nitrogen acquisition and metabolism. In addition, two genes encoding sigma factors of RNA polymerase (syc0015 and syc0953) and a gene encoding a response regulator (syc1148) were upregulated by nitrogen depletion, with 2.5-, 11.5-, and 7.6-fold increase in the transcript levels, respectively, and found to contain an NtcA-binding site in their promoters. The upregulation of syc1148 was abolished in an NtcA deletion mutant, whereas significant upregulation of syc0015 and syc0953 was observed even in the mutant upon nitrogen depletion, indicating involvement of an NtcA-independent as well as an NtcA-dependent mechanism in gene activation. In a mutant deficient in syc0953, the extent of upregulation of nblA, a representative of NtcA-activated genes, was reduced to a level similar to that in the NtcA mutant, indicating direct and indirect regulation of nblA by NtcA.
