Some mutations are due to the insertion of a transposable element.

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An important source of new mutations in many organisms is the insertion of movable DNA sequences into or near a gene. Such movable DNA sequences are called transposable elements or transposons. As we saw in Chapter 13, the genomes of virtually all organisms contain several types of transposable element, each present in multiple copies per genome.

Transposable elements were discovered by American geneticist Barbara McClintock in the 1940s (Fig. 14.11). She studied corn (maize) because genetic changes that affect pigment formation can be observed directly in the kernels. The normal color of maize kernels is purple. Each kernel consists of many cells, and if no mutations affecting pigmentation occur in a kernel, it will be uniformly purple. The kernel pigments are synthesized by several different enzymes in a metabolic pathway, and any of these enzymes can be rendered nonfunctional by mutations in their genes, including the insertion of transposable elements. Since McClintock’s work, we have learned that most transposable elements are segments of DNA a thousand or more base pairs long. When such a large piece of DNA inserts into a gene, it can interfere with transcription, cause errors in RNA processing, or disrupt the open reading frame. The result in the case of maize is that the cell is unable to produce pigment, and so the kernels will be yellow.

HOW DO WE KNOW?

FIG. 14.11

What causes sectoring in corn kernels?

BACKGROUND In the late 1940s, Barbara McClintock discovered what are now called transposable elements, DNA sequences that can move from one position to any other in the genome. She studied corn (Zea mays). Wild-type corn has purple kernels, resulting from expression of purple anthocyanin pigment (Fig. 14.11a). A mutant with yellow kernels results from lack of purple anthocyanin pigment. McClintock noticed that streaks of purple pigmentation could be seen in many yellow kernels (Fig. 14.11b). This observation indicated that the mutation causing yellow color was unstable and that the gene could revert to the normal purple color.

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HYPOTHESIS McClintock hypothesized that the yellow mutant color resulted from a transposable element, which she called Dissociator (Ds), jumping into a site near or in the anthocyanin gene and disrupting its function. She attributed the purple streaks to cell lineages in which the transposable element had jumped out again, restoring the anthocyanin gene.

EXPERIMENT AND RESULTS By a series of genetic crosses, McClintock showed that the genetic instability of Ds was due to something on another chromosome that she called Activator (Ac). She set up crosses in which she could track the Ac-bearing chromosome. She observed that in the presence of Ac, cells in mutant yellow kernels reverted to normal purple, resulting in purple sectors in an otherwise yellow kernel. From this observation, she inferred that the Ds element had jumped out of the anthocyanin gene, restoring its function. She also demonstrated that restoration of the original purple color was associated with mutations elsewhere in the genome. From this observation, she inferred that the Ds element had integrated elsewhere in the genome, where it disrupted the function of another gene.

CONCLUSION McClintock’s conclusion is illustrated in Fig. 14.11c: Transposable elements can be excised from their original position in the genome and inserted into another position.

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FIG. 14.11

FOLLOW-UP WORK McClintock won the Nobel Prize in Physiology or Medicine in 1983. Later experiments showed that Ds is a transposable element that lacks a functional gene for transposase, the protein needed for the element to move, and Ac is a transposable element that encodes transposase. Presence of Ac produces active transposase that allows Ds to move. Much additional work showed that there are many different types of transposable element and that they are ubiquitous among organisms.

SOURCE McClintock, B. 1950. “The Origin and Behavior of Mutable Loci in Maize.” Proceedings of the National Academy of Sciences of the USA. 36:344–355.

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Among mutants affecting kernel pigmentation, McClintock observed certain mutants that resulted in kernels that were mostly yellow but speckled with purple. She suspected that these particular mutants might result from a transposable element jumping into and out of a gene. Each colored sector consists of a lineage of daughter cells from a single ancestral cell in which pigment synthesis had been restored. McClintock realized that, just as the inability of the kernel cells to produce pigment was caused by a transposable element jumping into a gene, restoration of that ability could be caused by the transposable element jumping out again. Her hypothesis that transposable elements are responsible for the pigment mutations was confirmed when she found that in cells where pigment had been restored, mutations affecting other genes had also occurred. She deduced that these other mutations were due to a transposable element jumping out of a pigment gene and into a different gene in the same cell.

The movement, or transposition, of transposable elements occurs by different mechanisms according to the type of transposon (Chapter 13). McClintock’s original discovery was of a DNA transposon that transposes by a cut-and-paste mechanism in which the transposon is cleaved from its original location in the genome by a specific enzyme (transposase) and inserted into a different position. Removal of the transposon from its original position and repair of the cleavage leads to restoration of gene function, which in McClintock’s experiment was the ability to produce purple pigment. Unstable mutations due to DNA transposons can occur in almost any organism, including the Japanese morning glory, whose sectored flowers are shown in Fig. 14.3.

Not all transposons undergo transposition by a cut-and-paste mechanism. As discussed in Chapter 13, retrotransposons undergo transposition by an RNA intermediate, and when these types of transposable elements move, the retrotransposon used as a template for transcription stays behind in its original location. This mode of transposition might be called a copy-and-paste mechanism. It is mediated by two enzymes. One is reverse transcriptase (Chapter 13), which produces a double-stranded DNA copy of the retrotransposon from its RNA transcript, much in the same way that reverse transcriptase produces double-stranded DNA from viral RNA genomes. The other enzyme is an integrase, which cuts genomic DNA and inserts the retrotransposon at the cut site. By this mechanism, a retrotransposon can be copied and pasted at various sites in the genome, potentially disrupting the function of many different genes.