“…A hypothesis consistent with the data is that the cell is using the SITs to monitor whether sufficient silicic acid is present to complete the formation of a cell wall component, particularly the valve. Alternatively, another silicon-sensing protein upstream of silicon transport could be involved; however, the SITs are the only proteins yet identified to specifically interact with silicic acid (15), and there is precedence for transporters functioning as sensors (58,59). The TAG accumulation results (Fig.…”
The utilization of silicon by diatoms has both global and small-scale implications, from oceanic primary productivity to nanotechnological applications of their silica cell walls. The sensing and transport of silicic acid are key aspects of understanding diatom silicon utilization. At low silicic acid concentrations (<30 M), transport mainly occurs through silicic acid transport proteins (SITs), and at higher concentrations it occurs through diffusion. Previous analyses of the SITs were done either in heterologous systems or without a distinction between individual SITs. In the present study, we examined individual SITs in Thalassiosira pseudonana in terms of transcript and protein abundance in response to different silicic acid regimes and examined knockdown lines to evaluate the role of the SITs in transport, silica incorporation, and lipid accumulation resulting from silicon starvation. SIT1 and SIT2 were localized in the plasma membrane, and protein levels were generally inversely correlated with cellular silicon needs, with a distinct response being found when the two SITs were compared. We developed highly effective approaches for RNA interference and antisense knockdowns, the first such approaches developed for a centric diatom. SIT knockdown differentially affected the uptake of silicon and the incorporation of silicic acid and resulted in the induction of lipid accumulation under silicon starvation conditions far earlier than in the wild-type cells, suggesting that the cells were artificially sensing silicon limitation. The data suggest that the transport role of the SITs is relatively minor under conditions with sufficient silicic acid. Their primary role is to sense silicic acid levels to evaluate whether the cell can proceed with its cell wall formation and division processes.
Silicon, a member of group 14 of the periodic table containing carbon, germanium, and lead, is one of the most abundant elements (27.7%) of the Earth's crust second only to oxygen (46.6%) (1). Dissolved silica in the form of orthosilicic acid, Si(OH) 4 , derived from the weathering of the Earth's crust (2), is an essential nutrient for a few groups of organisms of marine and freshwater environments, such as diatoms, radiolaria, silicoflagellates, and sponges (3). Diatoms require a large quantity of silicon, which is biomineralized to form their unique siliceous cell wall (frustule) in various shapes and morphologies with highly intricate and ornate architectures. Diatoms contribute about 40% of the total ocean organic carbon production, and they dominate ocean Si production (4, 5). Though silicon does not appear to be directly involved in cellular metabolic processes, diatoms require silicon for progression of cell cycle events, such as cell division and DNA replication (5-8). Silicon requirements are tightly coupled to cell cycle progression, a unique feature of diatoms (7). Deprivation of Si arrests the cell cycle at G 1 , G 2 , or M phase, depending upon the diatom species. In Thalassiosira pseudonana, upon Si depletion the ...
“…A hypothesis consistent with the data is that the cell is using the SITs to monitor whether sufficient silicic acid is present to complete the formation of a cell wall component, particularly the valve. Alternatively, another silicon-sensing protein upstream of silicon transport could be involved; however, the SITs are the only proteins yet identified to specifically interact with silicic acid (15), and there is precedence for transporters functioning as sensors (58,59). The TAG accumulation results (Fig.…”
The utilization of silicon by diatoms has both global and small-scale implications, from oceanic primary productivity to nanotechnological applications of their silica cell walls. The sensing and transport of silicic acid are key aspects of understanding diatom silicon utilization. At low silicic acid concentrations (<30 M), transport mainly occurs through silicic acid transport proteins (SITs), and at higher concentrations it occurs through diffusion. Previous analyses of the SITs were done either in heterologous systems or without a distinction between individual SITs. In the present study, we examined individual SITs in Thalassiosira pseudonana in terms of transcript and protein abundance in response to different silicic acid regimes and examined knockdown lines to evaluate the role of the SITs in transport, silica incorporation, and lipid accumulation resulting from silicon starvation. SIT1 and SIT2 were localized in the plasma membrane, and protein levels were generally inversely correlated with cellular silicon needs, with a distinct response being found when the two SITs were compared. We developed highly effective approaches for RNA interference and antisense knockdowns, the first such approaches developed for a centric diatom. SIT knockdown differentially affected the uptake of silicon and the incorporation of silicic acid and resulted in the induction of lipid accumulation under silicon starvation conditions far earlier than in the wild-type cells, suggesting that the cells were artificially sensing silicon limitation. The data suggest that the transport role of the SITs is relatively minor under conditions with sufficient silicic acid. Their primary role is to sense silicic acid levels to evaluate whether the cell can proceed with its cell wall formation and division processes.
Silicon, a member of group 14 of the periodic table containing carbon, germanium, and lead, is one of the most abundant elements (27.7%) of the Earth's crust second only to oxygen (46.6%) (1). Dissolved silica in the form of orthosilicic acid, Si(OH) 4 , derived from the weathering of the Earth's crust (2), is an essential nutrient for a few groups of organisms of marine and freshwater environments, such as diatoms, radiolaria, silicoflagellates, and sponges (3). Diatoms require a large quantity of silicon, which is biomineralized to form their unique siliceous cell wall (frustule) in various shapes and morphologies with highly intricate and ornate architectures. Diatoms contribute about 40% of the total ocean organic carbon production, and they dominate ocean Si production (4, 5). Though silicon does not appear to be directly involved in cellular metabolic processes, diatoms require silicon for progression of cell cycle events, such as cell division and DNA replication (5-8). Silicon requirements are tightly coupled to cell cycle progression, a unique feature of diatoms (7). Deprivation of Si arrests the cell cycle at G 1 , G 2 , or M phase, depending upon the diatom species. In Thalassiosira pseudonana, upon Si depletion the ...
“…Basically, bacteria use three major types: membrane-integrated one-component systems (ToxR-like receptors), two-component systems consisting of a receptor histidine kinase and a response regulator, and the so-called extracytoplasmic function (ECF) sigma factors [ 5 ] ( figure 1 ). In some cases, membrane-integrated transport proteins have secondarily acquired sensory functions [ 6 , 7 ]. Figure 1.…”
Every living cell possesses numerous transmembrane signalling systems that receive chemical and physical stimuli from the environment and transduce this information into an intracellular signal that triggers some form of cellular response. As unicellular organisms, bacteria require these systems for survival in rapidly changing environments. The receptors themselves act as ‘sensory organs’, while subsequent signalling circuits can be regarded as forming a ‘neural network’ that is involved in decision making, adaptation and ultimately in ensuring survival. Bacteria serve as useful biosensors in industry and clinical diagnostics, in addition to producing drugs for therapeutic purposes. Therefore, there is a great demand for engineered bacterial strains that contain transmembrane signalling systems with high molecular specificity, sensitivity and dose dependency. In this review, we address the complexity of transmembrane signalling systems and discuss principles to rewire receptors and their signalling outputs.
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