Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

806 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
Chloroplasts and Bacteria Share Many Striking Similarities
The chloroplast genomes of land plants range in size from 70,000 to 200,000 nucleotide
pairs. More than 300 chloroplast genomes have now been sequenced. Many
are surprisingly similar, even in distantly related plants (such as tobacco and liverwort),
and even those of green algae are closely related (Figure 14–67). Chloroplast
genes are involved in three main processes: transcription, translation, and
photosynthesis. Plant chloroplast genomes typically encode 80–90 proteins and
around 45 RNAs, including 37 or more tRNAs. As in mitochondria, most of the
organelle-encoded proteins are part of larger protein complexes that also contain
one or more subunits encoded in the nucleus and imported from the cytosol.
The genomes of chloroplasts and bacteria have striking similarities. Basic regulatory
sequences, such as transcription promoters and terminators, are virtually
identical. The amino acid sequences of the proteins encoded in chloroplasts are
clearly recognizable as bacterial, and several clusters of genes with related functions
(such as those encoding ribosomal proteins) are organized in the same way
in the genomes of chloroplasts, the bacterium E. coli, and cyanobacteria.
The mechanisms by which chloroplasts and bacteria divide are also similar.
Both utilize FtsZ proteins, which are self-assembling GTPases related to tubulins
(see Chapter 16). Bacterial FtsZ is a soluble protein that assembles into a dynamic
ring of membrane-attached protofilaments beneath the plasma membrane in the
middle of the dividing cell. The FtsZ ring acts as a scaffold for recruitment of other
cell-division proteins and generates a contractile force that results in membrane
constriction and eventually in cell division. Presumably, chloroplasts divide in
very much the same way. Although both employ membrane-interacting GTPases,
the mechanisms by which mitochondria and chloroplasts divide are fundamentally
different. The machinery for chloroplast division acts from the inside, as in
bacteria, while the dynamin-like GTPases divide mitochondria from the outside
(see Figure 14–63). The chloroplasts have remained closer to their bacterial origins
than have mitochondria, since the eukaryotic mechanisms of membrane
constriction and vesicle formation have been adapted for mitochondrial fission.
The RNA editing and RNA processing that is prevalent in chloroplasts owes
everything to their eukaryotic hosts. This RNA processing includes the generation
of transcript 5ʹ and 3ʹ termini and the cleavage of polycistronic transcripts.
In addition, an RNA editing process converts specific C residues to U and can
change the amino acid specified by the edited codon. These and other RNA-based
23S
23S
16S
liverwort chloroplast DNA
121,024 bp
16S
KEY:
tRNA genes
ribosomal protein genes
photosystem I genes
photosystem II genes
ATP synthase genes
genes for b 6 -f complex
RNA polymerase genes
genes for NADH
dehydrogenase complex
ribulose bisphosphate
carboxylase (large
subunit)
inverted repeats
containing ribosomal
RNA genes
Figure 14–67 The organization
of the liverwort chloroplast
genome. The chloroplast
genome organization is similar
in all higher plants, although
the size varies from species to
species—depending on how
much of the DNA surrounding the
genes encoding the chloroplast’s
16S and 23S ribosomal RNAs is
present in two copies.