9. The chloroplast genome codes for 100 proteins and RNAs

3.9 The chloroplast genome codes for ~100 proteins and RNAs




Figure 3.14 The chloroplast genome codes for 4 rRNAs, 30 tRNAs, and ~50 proteins.

What genes are carried by chloroplasts? Figure 3.14 summarizes a situation generally similar to that of mitochondria, except that more genes are involved. The chloroplast genome codes for all the rRNA and tRNA species needed for protein synthesis. The ribosome includes two small rRNAs in addition to the major species. The tRNA set resembles that of mitochondria in including fewer species than would suffice in the cytoplasm. The chloroplast genome codes for ~50 proteins, including RNA polymerase and some ribosomal proteins. Again the rule is that organelle genes are transcribed and translated by the apparatus of the organelle.


The complete sequence of chloroplast DNA has been determined for a liverwort (a moss) and for tobacco. In spite of a considerable difference in overall length, between 121 kb and 155 kb, the gene organization is similar, and the overall number of genes almost identical. Most of their products are components of the thylakoid membranes or concerned with redox reactions, as can be seen from Figure 3.14.


Introns in chloroplasts fall into two general classes. Those in tRNA genes are usually (although not inevitably) located in the anticodon loop, like the introns found in yeast nuclear tRNA genes (which we discuss in 22 Nuclear splicing and RNA processing). Those in protein-coding genes resemble the introns of mitochondrial genes (see 23 Catalytic RNA). This places the endosymbiotic event at a time in evolution before the separation of prokaryotes with uninterrupted genes.


The role of the chloroplast is to undertake photosynthesis. Many of its genes code for proteins of complexes located in the thylakoid membranes. The constitution of these complexes shows a different balance from that of mitochondrial complexes. Although some complexes are like mitochondrial complexes in having some subunits coded by the organelle genome and some by the nuclear genome, other chloroplast complexes are coded entirely by one genome.


The identified genes of the chloroplast show the focus of its activities. There are 45 genes coding for RNA, 27 coding for proteins concerned with gene expression, 18 coding for proteins of the thylakoid membrane, and another 10 representing functions concerned with electron transfer. The products of some 30 open reading frames remain to be identified (Spoor et al., 1999; Shimada et al., 1991; for review see Palmer, 1985).




Reviews
Palmer, J. D. (1985). Comparative organization of chloroplast genomes. Ann. Rev. Genet. 19, 325-354.
Shimada H, et al. (1991). Fine structural features of the chloroplast genome: comparison of the sequenced chloroplast genomes. Nucleic Acids Res 11, 983-995.

Research
Spoor, F. , O'Higgins, P. , Dean, C. , and Lieberman, D. E. (1999). Anterior sphenoid in modern humans . Nature 397, 572-574.



Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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