Interestingly, in many species, one or two tRNAs for mitochondrial protein synthesis are encoded in the nucleus. Some parasites, for example trypanosomes, have not retained any tRNA genes in their mitochondrial DNA. Instead, the required tRNAs are all produced in the cytosol and are thought to be imported into the mitochondrion by special tRNA translocases that are distinct from the mitochondrial protein import system.
Figure 14–66 Comparison of various sizes of mitochondrial genomes with the genome of bacterial ancestors. the complete DNa sequences for thousands of mitochondrial genomes have been determined. the lengths of a few of these mitochondrial DNas are shown to scale— as circles for those genomes thought to be circular and lines for linear genomes. the largest circle represents the genome of Rickettsia prowazekii, a small pathogenic bacterium whose genome most closely resembles that of mitochondria. the size of mitochondrial genomes does not correlate well with the number of proteins encoded in them: while human mitochondrial DNa encodes 13 proteins, the 22-fold larger mitochondrial DNa of Arabidopsis thaliana encodes only 32 proteins—that is, about 2.5-fold as many as human mitochondrial DNa. the extra DNa that is found in Arabidopsis, Marchantia, and other plant mitochondria may be “junk DNa”—that is, noncoding DNa with no apparent function. the mitochondrial DNa of the protozoan Reclinomonas americana has 98 genes. (adapted from M.W. Gray, G. Burger and B.F. Lang, Science 283:1476–1481, 1999.)
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.