Rudiments of Prokaryotic Cell Structure
A typical prokaryote, E. coli, is a rod capped with hemispheres (Fig. 1.1). It is 1–3 µ (10-4 cm = 1 µ = 104 Å) long and 0.75 µ in diameter. Such a cell contains about 2 × 10-13 g of protein, 2 × 10-14 g of RNA that is mostly ribosomal RNA, and 6 × 10-15 g of DNA.
The cell envelope consists of three parts, an inner and outer mem-brane and an intervening peptidoglycan layer (Fig. 1.2). The outer surface of the outer membrane is largely lipopolysaccharides. These are attached to lipids in the outer half of the outer membrane. The polysac-charides protect the outer membrane from detergent-like molecules found in our digestive tract.outer membrane The outer membrane also consists of matrix proteins that form pores small enough to exclude the detergent-like bile salts, but large enough to permit passage of small molecules and phospholipids.
Figure 1.3 Structure of the cell wall showing the alternating N-acetylglu-cosamine N-acetylmuramic acid units. Each N-acetylmuramic acid possesses a peptide, but only a few are crosslinked in E. coli.
The major shape-determining factor of cells is the peptidoglycan layer or cell wall (Fig. 1.3). It lies beneath the outer membrane and is a single molecule containing many polysaccharide chains crosslinked by short peptides (Fig. 1.4). The outer membrane is attached to the pepti-doglycan layer by about 106 lipoprotein molecules. The protein end of each of these is covalently attached to the diaminopimelic acid in the peptidoglycan. The lipid end is buried in the outer membrane.
The innermost of the three cell envelope layers is the inner or cytoplasmic membrane. It consists of many proteins embedded in a phospholipid bilayer. The space between the inner membrane and the outer membrane that contains the peptidoglycan layer is known as the periplasmic space. The cell wall and membranes contain about 20% of the cellular protein. After cell disruption by sonicating or grinding, most of this protein is still contained in fragments of wall and membrane and can be easily pelleted by low-speed centrifugation.
The cytoplasm within the inner membrane is a protein solution at about 200 mg/ml, about 20 times more concentrated than the usual cell-free extracts used in the laboratory. Some proteins in the cytoplasm may constitute as little as 0.0001% by weight of the total cellular protein whereas others may be found at levels as high as 5%. In terms of concentrations, this is from 10-8 M to 2 × 10-4 M, and in a bacterial cell this is from 10 to 200,000 molecules per cell. The concentrations of many of the proteins vary with growth conditions, and a current re-search area is the study of the cellular mechanisms responsible for the variations.
Figure 1.4 Structure of the peptide crosslinking N-acetylmuramic acid units. DAP is diaminopimelic acid
The majority of the more than 2,000 different types of proteins found within a bacterial cell are located in the cytoplasm. One question yet to be answered about these proteins is how they manage to exist in the cell without adhering to each other and forming aggregates since polypep-tides can easily bind to each other. Frequently when a bacterium is engineered for the over-synthesis of a foreign protein, amorphous precipitates called inclusion bodies form in the cytoplasm. Sometimes these result from delayed folding of the new protein, and occasionally they are the result of chance coprecipitation of a bacterial protein and the newly introduced protein. Similarly, one might also expect an occasional mutation to inactivate simultaneously two apparently unre-lated proteins by the coprecipitation of the mutated protein and some other protein into an inactive aggregate, and occasionally this does occur.
The cell’s DNA and about 10,000 ribosomes also reside in the cyto-plasm. The ribosomes consist of about one-third protein and two-thirds RNA and are roughly spherical with a diameter of about 200 Å. The DNA in the cytoplasm is not surrounded by a nuclear membrane as it is in the cells of higher organisms, but nonetheless it is usually confined to a portion of the cellular interior. In electron micrographs of cells, the highly compacted DNA can be seen as a stringy mass occupying about one tenth of the interior volume, and the ribosomes appear as granules uniformly scattered through the cytoplasm.
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