Processing of Eukaryotic RNAs

The process of eukaryotic DNA replication closely follows that of prokaryotic DNA synthesis. Some differences, such as the multiple origins of replication in eukaryotic cells versus single origins of replication in prokaryotes, have already been discussed. Eukaryotic single-stranded DNA-binding proteins and ATP-dependent DNA helicases have been identified, whose functions are analogous to those of the prokaryotic enzymes previously discussed. In contrast, RNA primers are removed by RNase H. 

Rosen, J. M. Synthesis and processing of eukaryotic messenger RNA. In Receptors and Hormone Action, Vol. 1,... 

The process of synthesizing RNA from the genetic information encoded by DNA is called transcription. The enzymes involved in transcription are called RNA pol)unerases. Prokaryotes have one type eukaryotes have three types of nuclear RNA polymerases. 

Nascently transcribed eukaryotic iRNAs are among the most processed of all RNA polymerase III transcripts. Like those of prokaryotic tRNAs, the 5 leader is cleaved by RNase P, the 3 trailer is removed, and CCA is added by the CCA-adding enzyme (Figure 29.25). Eukaryotic tRNAs are also heavily modified on base and ribose moieties these modifications are important for function. In contrast with prokaryotic tRNAs, many eukaryotic pre-tRNAs are also spliced by an endonuclease and a ligase to remove an intron. 

The versatility of RNA first became clear from observations of the processing of ribosonial RNA in a single-cell eukaryote. In Tetrahymena (a ciliated protozoan), a 414-nucleotide intron is removed from a 6.4-kb precursor to yield the mature 26S rRNA molecule (Figure 29.37). In an elegant series of studies of this splicing reaction, Thomas Cech and his coworkers established that the RNA spliced itself to precisely excise the intron. These remarkable experiments demonstrated that an RNA molecule can splice itself in the absence of protein. Indeed, the RNA alone is catalytic and, under certain conditions, is thus a rihozyme. More than 1500 similar in-trons have since been found in species as widely dispersed as bacteria and eukaryotes, though not in vertebrates. Collectively, they are referred to as group I introns. 

The primary transcripts generated by RNA polymerase II—one of three distinct nuclear DNA-depen-dent RNA polymerases in eukaryotes—are promptly capped by 7-methylguanosine triphosphate caps (Figure 35-10) that persist and eventually appear on the 5 end of mature cytoplasmic mRNA. These caps are necessary for the subsequent processing of the primary transcript to mRNA, for the translation of the mRNA, and for protection of the mRNA against exonucleolytic attack. 

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