Small secreted proteins enable biofilm development in the cyanobacterium Synechococcus elongatus.

Small proteins characterized by a double-glycine (GG) secretion motif, typical of secreted bacterial antibiotics, are encoded by the genomes of diverse cyanobacteria, but their functions have not been investigated to date. Using a biofilm-forming mutant of Synechococcus elongatus PCC 7942 and a mutational approach, we demonstrate the involvement of four small secreted proteins and their GG-secretion motifs in biofilm development. These proteins are denoted EbfG1-4 (enable biofilm formation with a GG-motif). Furthermore, the conserved cysteine of the peptidase domain of the Synpcc7942_1133 gene product (dubbed PteB for peptidase transporter essential for biofilm) is crucial for biofilm development and is required for efficient secretion of the GG-motif containing proteins. Transcriptional profiling of ebfG1-4 indicated elevated transcript levels in the biofilm-forming mutant compared to wild type (WT). However, these transcripts decreased, acutely but transiently, when the mutant was cultured in extracellular fluids from a WT culture, and biofilm formation was inhibited. We propose that WT cells secrete inhibitor(s) that suppress transcription of ebfG1-4, whereas secretion of the inhibitor(s) is impaired in the biofilm-forming mutant, leading to synthesis and secretion of EbfG1-4 and supporting the formation of biofilms

Inactivation of the extrinsic subunit of photosystem II, PsbU, in Synechococcus PCC 7942 results in elevated resistance to oxidative stress

AbstractPsbU is a subunit of the extrinsic complex attached to the core of photosystem II. A PsbU-mutant of Synechococcus PCC 7942 was isolated based on its elevated resistance to externally applied oxidative stress. PsbU-mutant exhibits fast rates of degradation of the photosystem II core protein, D1, under sub-saturating as well as high-light conditions. While forward electron transfer is not affected, back electron flow is severely impaired in the mutant. We suggest that impairment of psbU results in production of reactive-oxygen-species, which trigger antioxidative mechanisms even under standard growth conditions. Accordingly, when challenged with external oxidative stress, these cells are more resistant than wild type cells

Oxidative Stress in Synechococcus sp. Strain PCC 7942: Various Mechanisms for H(2)O(2) Detoxification with Different Physiological Roles

This study focuses on the mechanisms for hydrogen peroxide detoxification in Synechococcus sp. strain PCC 7942. To gain better understanding of the role of different routes of hydrogen peroxide detoxification, we inactivated tplA (thioredoxin-peroxidase-like), which we recently identified. In addition, we inactivated the gene encoding catalase-peroxidase and examined the ability to detoxify H(2)O(2) and to survive oxidative stress in both of the single mutants and in the double mutant. Surprisingly, we observed that the double mutant survived H(2)O(2) concentrations that the single catalase-peroxidase mutant could not tolerate. This phenotype correlated with an increased ability of the double mutant to detoxify externally added H(2)O(2) compared to the catalase-peroxidase mutant. Therefore, our studies suggested the existence of a hydrogen peroxide detoxification activity in addition to catalase-peroxidase and thioredoxin-peroxidase. The rate of detoxification of externally added H(2)O(2) was similar in the wild-type and the TplA mutant cells, suggesting that, under these conditions, catalase-peroxidase activity was essential for this process and TplA was dispensable. However, during excessive radiation, conditions under which the cell might experience oxidative stress, TplA appears to be essential for growth, and cells lacking it cannot compete with the wild-type strain. Overall, these studies suggested different physiological roles for various cellular hydrogen peroxide detoxification mechanisms in Synechococcus sp. strain PCC 7942

Quantification of Chlorophyll as a Proxy for Biofilm Formation in the Cyanobacterium Synechococcus elongatus.

A self-suppression mechanism of biofilm development in the cyanobacterium Synechococcus elongatus PCC 7942 was recently reported. These studies required quantification of biofilms formed by mutants impaired in the biofilm-inhibitory process. Here we describe in detail the use of chlorophyll measurements as a proxy for biomass accumulation in sessile and planktonic cells of biofilm-forming strains. These measurements allow quantification of the total biomass as estimated by chlorophyll level and representation of the extent of biofilm formation by depicting the relative fraction of chlorophyll in planktonic cells

Impairment of a cyanobacterial glycosyltransferase that modifies a pilin results in biofilm development.

A biofilm inhibiting mechanism operates in the cyanobacterium Synechococcus elongatus. Here, we demonstrate that the glycosyltransferase homologue, Ogt, participates in the inhibitory process - inactivation of ogt results in robust biofilm formation. Furthermore, a mutational approach shows requirement of the glycosyltransferase activity for biofilm inhibition. This enzyme is necessary for glycosylation of the pilus subunit and for adequate pilus formation. In contrast to wild-type culture in which most cells exhibit several pili, only 25% of the mutant cells are piliated, half of which possess a single pilus. In spite of this poor piliation, natural DNA competence was similar to that of wild-type; therefore, we propose that the unglycosylated pili facilitate DNA transformation. Additionally, conditioned medium from wild-type culture, which contains a biofilm inhibiting substance(s), only partially blocks biofilm development by the ogt-mutant. Thus, we suggest that inactivation of ogt affects multiple processes including production or secretion of the inhibitor as well as the ability to sense or respond to it