"Frontmatter". In: Plant Genomics and Proteomics
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Christopher A. Cullis - Plant Genomics and Proteomics-J. Wiley & Sons (2004)
CHAPTER
5 C O N T R O L O F G E N E E X P R E S S I O N O VERVIEW The control of gene expression can occur at many levels (Figure 5.1). The primary level involves the transcription of the DNA into RNA. There are many controls over the initiation of this event including the need for pro- moters and terminator sequences, for appropriate chromatin structure and the presence of enhancers or modulating sequences that affect the availabil- ity of a particular gene for transcription. Once transcribed the RNA then must be processed into a mature message and exported from the nucleus in a form that permits translation by the ribosomal machinery. The half-lives of various RNAs, and even their capability of being translated, differ widely. After translation the RNAs can either be recycled and translated again or can be targeted for degradation. The final points of control are at the protein level with protein modification events that affect the ultimate function of the protein product of the gene. Not all genes are turned on at all times during the life cycle of a plant. Different genes are required for the completion of different steps in the devel- opmental and sexual maturation of the plant and in response to the external environment (see Chapter 7). Two general types of control can be described: ∑ Temporal regulation—when a gene is only expressed at a specific time in development, for example, genes that are only expressed in the light or during flower development ∑ Spatial regulation—when a gene is only expressed in a specific loca- tion in the plant, for example, seed storage proteins or anther specific genes Plant Genomics and Proteomics, by Christopher A. Cullis ISBN 0-471-37314-1 Copyright © 2004 John Wiley & Sons, Inc. 8 9 Gene expression is controlled by both the sequences surrounding the gene and the chromosome context in which the gene resides. The promoter elements can be complex containing sequences that control temporal and spatial regulation as well as enhancer and silencing activities. Added to these 9 0 5. C O N T R O L O F G E N E E X P R E S S I O N chromatin exon intron DNA Transcription pre-mRNA mRNA Processing transport Protein modification Protein degradation AAAAAA RNA degradation translation 5' 5' nuclear membrane FIGURE 5.1. Points of control in gene expression. The region of the chromosome to be transcribed accumulates RNA polymerase. The nascent RNA is processed and then transported to the cytoplasm to be translated. The RNA is degraded or reused. The peptide is folded and/or modified to deliver the final product that is itself degraded at some point. elements, the role of 3¢ sequences and introns in gene expression also must be considered. Chromatin variation brought about by the reversible modifi- cation of the core histones by acetylation, methylation, phosphorylation, ubiquitination, or ADP-ribosylation (Strahl and Allis, 2000) all have conse- quences for gene activation, gene repression, and chromosome replication. Epigenetic phenomena are alternative states of gene expression, molecular function, or organization specified by the same genetic instructions (the primary DNA sequence). Chromatin modifications including cytosine methylation and histone modifications are known to be involved in many such epigenetic phenomena (Habu et al., 2001). Many of the experiments to determine the nature of control elements have been done by making transgenic plants. Unfortunately, high-efficiency homologous recombination is not currently available in plants so that it is not possible to place such control elements in their appropriate chromoso- mal context. This raises concerns because it is clear that the chromosomal context counts for the appropriate expression patterns to be reproduced, so that the expression patterns observed with transgenic plants may not accu- rately reflect the normal in vivo situation (Li et al., 2002). Download 1.13 Mb. Do'stlaringiz bilan baham: |
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