In Salmonella, the flagella can be made from two alternative proteins, H1 and H2. The genes for H2 protein and the rh1 repressor lie downstream of the promoter on the invertible segment (Fig. 13.12). When these are expressed, rh1 represses the gene for H1 protein, which is located elsewhere on the bacterial chromosome. When the segment inverts, due to Hin invertase, the H2 and rh1 genes are no longer expressed. Lack of the rh1 repressor allows instead expression of the H1 gene. Thus either H1 or H2 is expressed, but never both at once. These flagellar proteins (known historically as H-antigens) are strong antigens and found on the surface of bacteria. Periodically switching at random between different flagellar proteins helps bacteria avoid detection by the mammalian immune system.
Pathogenic strains of E. coli often bind to intestinal cells by thin helical filaments of protein known as pili or fimbriae. These are also strong surface antigens and are often subject to phase variation by a similar mechanism as described above. Appropriately enough, certain viruses that infect enteric bacteria also use phase variation to change the tail fiber proteins that recognize bacterial surface proteins during infection. This phase variation results in different host ranges for the alternative types of virus particle. For example, bacteriophage Mu flip-flops between recognizing E. coli and the closely related Citrobacter.
Many mutations have no effect on the phenotype—they are "silent".
A silent mutation is an alteration in the DNA sequence that has no effect on the operation of the cell and is therefore not so much silent as invisible from the outside. In other words, silent mutations do not alter the phenotype. One obvious kind of silent mutation is a base change occurring in the non-coding DNA between genes. Therefore, no genes are damaged and no proteins are altered. Higher organisms possess intervening sequences, the introns, within many of their genes. Since introns are cut out and discarded when the messenger RNA is made, most alterations to the sequence of an intron will not affect the final protein.
Not all base changes in an intron are harmless. Changes in the few critical bases at the splice recognition sites will result in failure to splice out the intron or in aberrant splicing. This will give a severely damaged protein product when the misspliced mRNA is translated. Further, many of the small nucleolar RNAs are derived from introns in other genes (Ch. 12). In addition, occasional cases are known where an intron silent mutation An alteration in the DNA sequence that has no effect on the phenotype
Silent Mutations Do Not Alter the Phenotype 347
FIGURE 13.12 Phase Variation of Flagella in Salmonella
The top diagram illustrates the state where flagellar protein H2 is made. The invertible segment has its promotor aligned to express H2 and the rh1 repressor, which then represses the gene for H1. Therefore in this orientation only H2 is made. In the bottom diagram the invertible segment is reversed. Neither H2 nor the repressor for H1 are made. Thus H1 is free to be expressed. The two forms of the gene are interconvertable.
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