5S gene

The gene encoding the smallest structural RNA component of the nuclear ribosome is the 5S rRNA gene. This gene is not co-located with the remaining nuclear rRNA genes. Neither sequence nor chromosomal location of the 5S gene is known for any neodermatan and it will not be discussed further here. 

Eukaryotic RNA polymerases are less well-characterized than the bacterial enzyme. Biochemical studies have so far identified three distinct nuclear RNA polymerase activities in eukaryotic cell extracts prepared from both yeast and human cells. These RNA polymerases, referred to as RNA pol I, RNA pol II, and RNA pol HI, each contain a large number of subunits, some of which appeared to be shared between the different RNA polymerase subtypes. As shown in Table 24.1, RNA pol I transcribes ribosomal RNA genes, RNA pol II transcribes the majority of protein-coding genes, into mRNA, and RNA pol III transcribes tRNA, small nuclear RNAs (snRNAs), and ribosomal 5S genes. Note also that the mitochondrial genome is transcribed by a nuclear-encoded mitochondrial RNA polymerase. The yeast mitochondrial RNA polymerase holoenzyme consists of a 140-kd catalytic subunit and a 43-kd promoter-recognition protein similar to the bacterial s factor. 

S Ribosomal RNA genes - The most thoroughly studied genes transcribed by RNA polymerase III are those for 5S ribosomal RNA. At least three protein factors in addition to polymerase III are needed for the expression of the 5S rRNA genes in vitro. Two of these transcription factors (TFIIIB and TFIIIC) appear to participate in the transcription of tRNA genes as well, but one (called TFIIIA) is specific for the 5S genes. 

RNA Polymerase III. The most intriguing data on the initiation of eukaryote transcription concerns that by RNA polymerase III. In vitro transcription systems for this enzyme have been used to define the initiation and termination sites for 5S RNA and tRNA synthesis. Brown and his co-workers have further employed them to look for control regions for the transcription of the 5S genes of Xenopus borealis. They deleted nucleotides from the 5 and the 3 side of the gene, cloned the resultant mutants and then tested their ability to support in vitro transcription. Many such deletions had no effect on transcription or its fidelity. Even when up to 50... 

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