Awakening of a Dormant Cyanobacterium from Nitrogen Chlorosis Reveals a Genetically Determined Program

Abstract

The molecular and physiological mechanisms involved in the transition of microbial cells from a resting state to the active vegetative state are critically relevant for solving problems in fields ranging from microbial ecology to infection microbiology. Cyanobacteria that cannot fix nitrogen are able to survive prolonged periods of nitrogen starvation as chlorotic cells in a dormant state. When provided with a usable nitrogen source, these cells re-green within 48 hr and return to vegetative growth. Here we investigated the resuscitation of chlorotic Synechocystis sp. PCC 6803 cells at the physiological and molecular levels with the aim of understanding the awakening process of a dormant bacterium. Almost immediately upon nitrate addition, the cells initiated a highly organized resuscitation program. In the first phase, they suppressed any residual photosynthetic activity and activated respiration to gain energy from glycogen catabolism. Concomitantly, they restored the entire translational apparatus, ATP synthesis, and nitrate assimilation. After only 12-16 hr, the cells re-activated the synthesis of the photosynthetic apparatus and prepared for metabolic re-wiring toward photosynthesis. When the cells reached full photosynthetic capacity after ∼48 hr, they resumed cell division and entered the vegetative cell cycle. An analysis of the transcriptional dynamics during the resuscitation process revealed a perfect match to the observed physiological processes, and it suggested that non-coding RNAs play a major regulatory role during the lifestyle switch in awakening cells. This genetically encoded program ensures rapid colonization of habitats in which nitrogen starvation imposes a recurring growth limitation.

Chlorosis as a Developmental Program in Cyanobacteria: The Proteomic Fundament for Survival and Awakening

“…This protein, Slr1535, also contains a family 38 domain, which specifies it as an α-mannosidase. These findings agree with glycogen metabolism being a key to the entire chlorosis and resuscitation process (11). We addressed the specific roles of carbon catabolic enzymes for the resuscitation of chlorotic cells in a parallel study (manuscript submitted).…”

“…The components of the CO2-fixing apparatus, namely the carbon-concentrating mechanism (Ccm) proteins and RuBisCO (RbcLS) subunits, are highly repressed during chlorosis at the transcriptome level, and carboxysome structures were not detected in electron micrographs (11). However, at the proteome level, the abundances of the corresponding proteins were higher than expected from these features.…”

“…Protein synthesis in long-term chlorotic cells is diminished to a barely detectable level (9), and electron micrographs reveal an almost complete absence of ribosomes in longterm chlorotic cells (11). At the level of the proteome of chlorotic cells, the least abundant ribosomal proteins set the upper limit for the abundance of assembled ribosomes.…”

“…When nitrogen starvation persists, the cells gradually turn down metabolism and enter a dormant-like state characterized by minimum residual photosynthesis and pigmentation, which allows survival of prolonged periods of time (9,10). During that stage, cells maintain a substantial amount of mRNA (11), in spite of highly reduced protein synthesis (9), suggesting that these transcripts are either not translated or translated very slowly.…”

“…Following the addition of a nitrogen source, the cells rapidly awaken from dormancy and return to vegetative growth (11). Analysis of transcriptome dynamics during resuscitation, along with physiological and electron microscopy studies have revealed first insights into a genetically determined program guiding resuscitation.…”