The chloroplast genome codes for many proteins and RNAs

KEY CONCEPTS:

• Chloroplast genomes vary in size, but are large enough to code for 50-100 proteins as well as the rRNAs and tRNAs.

 What genes are carried by chloroplasts? Chloroplast DNAs vary in length from 120-190 kb. The sequenced chloroplast genomes (>10 in total) have 87-183 genes (Shimada et al., 1991; for review see Palmer, 1985; Sugiura, Hirose, and Sugita, 1998). Figure 3.42 summarizes the functions coded by the chloroplast genome in land plants. There is more variation in the chloroplast genomes of algae.

The situation is 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 may include all of the necessary genes. The chloroplast genome codes for ~50 proteins, including RNA polymerase and ribosomal proteins. Again the rule is that organelle genes are transcribed and translated by the apparatus of the organelle.

About half of the chloroplast genes code for proteins involved in protein synthesis. The endosymbiotic origin of the chloroplast is emphasized by the relationships between these genes and their counterparts in bacteria. The organization of the rRNA genes in particular is closely related to that of a cyanobacterium, which pins down more precisely the last common ancestor between chloroplasts and bacteria.

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 (see 24.14 Yeast tRNA splicing involves cutting and rejoining). Those in protein-coding genes resemble the introns of mitochondrial genes (see 26 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.