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Parts
of
Flagella -
1. Basal body. In, gram negative bacteria the basal body consists of two sets of rings, a proximal and a distal set, connected by a rod.
Each set has two rings, and the four rings are caned the M (membrane) ring, the S (super membrane) ring, the P peptidoglycan) ring and the L (lipopolysaccharide) ring, from the inner to the, outer side.
The M ring is embedded in the inner membrane (plasma membranes). The S ring lies in the periplasmic space between the inner and outer membranes.
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The P ring is attached to the peptidoglycan layer of the cell wall.
The L ring is attached to the lipopolysaccharide, layer of the outer cell wan membrane.
The P and the L rings form a bearing for the rod to pass through the outer cell wall membrane.
In gram positive bacteria the outer set of rings is absent.
The M ring is embedded in the inner membrane, as in gram negative bacteria.
The S ring may be attached to the inside of the thick peptidoglycan layer.
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2. The hook connects the basal body with the shaft. The hook of the gram positive bacterial flagellum is longer than that, of the gram negative flagellum. In Clostridium sporogenes a 'grommet', which may act as a bearing, has been observed.
3. The main filament or shaft. In contrast to the eukaryote flagellum, the bacterial flagellum consists almost entirely of protein. This protein occurs in multiple units and is termed flagel1in.
The flagellin of Bacillus subtilis (strain 168) has been completely sequenced. It is a single/polypeptide chain of 304 amino acid residues and has a molecular weight of 32,600.
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Its hydrophobic amino acids are distributed at random, but the charged amino acids show asymmetry in distribution.
The NH2 terminal region is basic, the middle region acidic and the -COOH terminal region weakly acidic.
The amino acid sequence is unusual in that tryptophan and cysteine are absent.
And there is only a single tyrosine residue and two proline residues.
The absence of cysteine is probably correlated with the need for a protein whose conformation is not easily affected by changes in the environmental redox potential.
In a B. subtilis 168 mutant, a single amino acid substitution at position 233 (valine instead of alanine )results in straight instead of helical flagella. These flagella are non-motile
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The molecular weight of flagellin from), other bacterial species ranges from 30,000 to 60,000. Tryptophan and cysteine are absent, and the cyclic amino acids phenylalanine, tyrosine, histidine and proline are, present in small amounts.
The flagellin of some serotypes of Salmonella contains methyl lysine (E-N-methyl lysine). This unusual amino acid is found in histones from eukaryotic nuclei, but has not been reported elsewhere in bacteria. Methyl lysine is, however, not essential, and the flagella of mutants lacking this amino acid function normally.
In Salmonella the hook protein (MW 43,000) is antigenically distinct from the shaft flagellin (MW 56,000). It lacks methyl lysine, but contains more cyclic amino acids. A similar hook composition has been reported for E. coli.
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The bacterial flagellum may be up to 15μm long, but is only about 20-30 nm thick. It is thus about 1/10th the diameter of the eukaryote flagellum.
The diameter of the bacterial flagellum approximately corresponds to that of a single microtubule of the eukaryote flagellum.
The shaft and the hook are constructed of spherical or ovoid flagellin subunits about 5 nm in diameter. They are helically arranged to form cylindrical fibrils, which appear to be hollow in some cases at least according to one model,
II nearly longitudmal rows of subunits form the tube of the shaft the central space appears to be narrower that what would be formed by a tube consisting of11 rows of subunits.
It bas, therefore, been suggested that the subunits are not spherical but elongated (pine cone model). Such a structure would have a reduced internal space, but would appear to be composed of roughly spherical units from the outside.
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