Genes for tRNA

For example, E. coli has three identical genes for tRNA, each producing a tRNA with the anticodon... 

RNA polymerase III (RNA Pol III) is also located in the nucleoplasm. It transcribes the genes for tRNA, 5S rRNA, U6 snRNA, and the 7S RNA associated with the signal recognition particle (SRP) involved in the translocation of proteins across the endoplasmic reticulum membrane (see Topic H4). 

In prokaryotes, gene R. has only been detected in isolated cases, and the values are low. In Bacillus sub-lilis only 0.4% of the genome is redundant (e.g. 9-10 genes for rRNA and 40 genes for tRNA). 

Unlike prokaryotic cells, eukaryotes contain three DNA-dependent RNA polymerase activities. Polymerase I (or A) catalyses the synthesis of ribosomal RNA precursors. Polymerase II (or B) transcribes the structural genes for proteins and polymerase III (or C) transcribes the genes for tRNA and 5S RNA. Consequendy, it is expected that the activity of these enzymes will be controlled in different ways and modem techniques of genetic manipulation have enabled considerable strides in the study of putative control regions for each of them. It cannot be said, however, that a clear consensus has yet emerged for the binding to the DNA of any of the three enzymes. 

Enzymatic Synthesis.—The total synthesis by chemical and enzymatic methods of the structural gene for the precursor of a tyrosine suppressor tRNA from E. coli, a DNA duplex that is 126 nucleotides long, has been described. This, Khorana s Meisterwerk to date, represents the highest state of the art, and proceeds by methods which are already famous, and the touchstone of those who wish to follow.135... 

Transcription is catalyzed by DNA-dependent RNA polymerases. These act in a similar way to DNA polymerases (see p. 240), except that they incorporate ribonucleotides instead of deoxyribonucleotides into the newly synthesized strand also, they do not require a primer. Eukaryotic cells contain at least three different types of RNA polymerase. RNA polymerase I synthesizes an RNA with a sedimentation coef cient (see p. 200) of 45 S, which serves as precursor for three ribosomal RNAs. The products of RNA polymerase II are hnRNAs, from which mRNAs later develop, as well as precursors for snRNAs. Finally, RNA polymerase III transcribes genes that code for tRNAs, 5S rRNA, and certain snRNAs. These precursors give rise to functional RNA molecules by a process called RNA maturation (see p. 246). Polymerases II and III are inhibited by a-amanitin, a toxin in the Amanita phalloides mushroom. 

Chloroplasts come in various sizes and shapes, but all contain a small number of DNA molecules ranging in size from 120-160 kb. Complete sequences are known for DNA from chloroplasts of a liverwort (121,025 bp),246 tobacco (155,844 bp),247 maize (Zea mays),248 and other plants. The 140,387 bp DNA from maize chloroplasts is a circular molecule containing the genes for 23S, 16S, 5S, and 4.5S RNA, for 30 species of tRNA, and for 70 different proteins. Among them are subunits of RNA polymerase, NADH dehydrogenase, subunits of both PSI and PSII, rubisco (large subunit), cytochromes b and/, six subunits of ATP... 

---