The cytoplasm contains a large number of solute low- and high-molecular-weight substances, RNA and approximately 20 000 ribosomes per cell. Bacteria have 70S ribosomes comprising 30S and 50S subunits. Bacterial ribosomes function as the organelles for protein synthesis. The cytoplasm is also frequently used to store reserve substances (glycogen depots, polymerized metaphosphates, lipids).
■ The Most Important Bacterial Cytoplasmic Membrane Proteins ^^^m
Permeases Active transport of nutrients from outside to inside against a concentration gradient.
Biosynthesis Required for biosynthesis of the cell wall, e.g., its murein (see enzymes under "Cell wall" p. 152). The enzymes that contribute to the final murein biosynthesis steps are for the most part identical with the "penicillin-binding proteins" (PBPs).
Secretion Four secretion systems differing in structure and mode of action system proteins have been described to date. Proteins are moved out of the cell with the help of these systems. A common feature of all four is the formation of protein cylinders that traverse the cytoplasmic membrane and, in Gram-negative bacteria, the outer cell wall membrane as well. See p. 17 on the special relevance of the type III secretion system to virulence.
Sensor proteins Transmit information from the cell's environment into its inte-(also known as rior. The so-called receiver domain extends outward, the transsignal proteins) mitter domain inward. The transmission activity is regulated by the binding of signal molecules to a receiver module. In two-component systems, the transmitter module transfers the information to a regulator protein, activating its functional module. This regulator segment can then bind to specific gene sequences and activate or deactivate one or more genes (see also Fig. 1.4, p. 19).
Respiratory Occur in bacteria with aerobic metabolism. Aerobic respiration chain enzymes functions according to the same principles as cellular respiration in eurkaryotes.
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