RRNA and tRNA genes

RNA polymerase III transcribes 5S rRNA and tRNA genes. The promoter of these transcripts can actually be located inside the gene itself, in contrast to all the other promoters discussed earlier. See Figure 12-10. 

Most of the DNA sequences which are transcribed give rise to mRNA, which is subsequently translated into protein. However, the most abundant species of RNA are ribosomal RNA (rRNA) and transfer RNA (tRNA), which do not code for protein but function in the process of translation. They are formed by a high level of transcription of a relatively small number of genes (called rRNA and tRNA genes). In bacteria, transcription of all genes is brought about by the enzyme RNA polymerase. 

Would actinomycin D be expected to block transcription of rRNA and tRNA genes and those that code for protein products ... 

In the chloroplasts of land plants, besides rRNA and tRNA genes, the sprA gene for the plastid 218-bp RNA was reported. Targeted deletion of this gene has no eflFect on the phenotype of the plant in a growth chamber however, in the field the plants exhibit stunted growth. ... 

Related to the discipline of proteomics is that of structural genomics. The latter focuses upon the large-scale systematic study of gene product structure. While this embraces rRNA and tRNA,... 

Enzymes That Catalyze Amino Acid Biosynthesis Are Regulated at the Level of Transcription Initiation The trp Operon Is Also Regulated after the Initiation Point for Transcription Genes for Ribosomes Are Coordinately Regulated Control of rRNA and tRNA Synthesis by the rel Gene... 

The cap protects the 5 end of the primary transcript against attack by ribonu-cleases that have specificity for 3 5 phosphodiester bonds and so cannot hydrolyze the 5 5 bond in the cap structure. In addition, the cap plays a role in the initiation step of protein synthesis in eukaryotes. Only RNA transcripts from eukaryotic protein-coding genes become capped prokaryotic mRNA and eukaryotic rRNA and tRNAs are uncapped. 

Transcripts of 5S rRNAs and tRNAs generally start just before and terminate immediately after single genes amongst sulfothermophiles, while these stable RNA genes are often clustered in transcriptional units in the thermophilic methanogens and other archaea. 

The genomes of prokaryotic and eukaryotic cells differ in size. The genome of the bacterium E. coli contains approximately 3,000 genes. All of this bacterial DNA has a function it either codes for proteins, rRNA, and tRNA, or it serves to regulate the synthesis of these gene products. In contrast, the genome of the human haploid cell contains between 30,000 and 50,000 genes, 10 to 15 times the number in E. coli. The function of most of this extra DNA has not been determined (an issue considered in more detail in Chapter 15). 

Although diploidy and introns account for some of the difference between the DNA content of humans and bacteria, a large difference remains that is related to the greater complexity of the human organism. Bacterial cells have a single copy of each gene, called unique DNA, and they contain very little DNA that does not produce functional products. Eukaryotic cells contain substantial amounts of DNA that does not code for functional products (i.e., proteins or rRNA and tRNA). In addition, some genes that encode functional products are present in multiple copies. 

---