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

The impact of nitrogen deficiency on the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 and three nbl (non-bleaching) mutants (DeltanblAl, DeltanblA2 and DeltanblB) was investigated. The Deltanbl/A 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 Nb1A was present, the phycobilisomes were degraded to provide a nitrogen source for continued growth and metabolism. An important difference between the wild-type, mutant DeltanblB, and the DeltanblA mutants was seen in the rod linker proteins. Under nitrogen-deprivation condition, the L(R)33 and L(R)34.5 linker proteins were extensively degraded in the wild-type and DeltanblB mutant, but remained intact in the DeltanblA mutants.