Characterization of Synechocystis sp. strain PCC 6803 and Δnbl mutants under nitrogen-deficient conditions

Abstract.

The impact of nitrogen deficiency on the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 and three nbl (non-bleaching) mutants (ΔnblA1, ΔnblA2 and ΔnblB) was investigated. The ΔnblA mutants entered a non-dividing, dormant state soon after the initiation of nitrogen starvation. The cells became larger, the membrane system was disorganized, and ribosomes were found near the membranes much less frequently. Photosystem II (PSII) activity declined to approximately 10% of the wild-type level and the amount of D1 protein declined precipitously, despite adequate psbA transcription; PSI activity declined, but more slowly. Transcription from PSII (except psbA), PSI and phycobilisome genes was very low. Fluorescence at 77K indicated many partially assembled or unassembled phycobilisomes. The level of transcript accumulation increased to normal by 4 h after the readdition of nitrogen to the culture. When NblA was present, the phycobilisomes were degraded to provide a nitrogen source for continued growth and metabolism. An important difference between the wild-type, mutant ΔnblB, and the ΔnblA mutants was seen in the rod linker proteins. Under nitrogen-deprivation condition, the L_R33 and L_R34.5 linker proteins were extensively degraded in the wild-type and ΔnblB mutant, but remained intact in the ΔnblA mutants.