DNA-Dependent Synthesis of RNA:- Eukaryotic Cells Have Three Kinds of Nuclear RNA Polymerases
The transcriptional machinery in the nucleus of a eu karyotic cell is much more complex than that in bacte ria. Eukaryotes have three RNA polymerases, designated I, II, and III, which are distinct complexes but have certain subunits in common. Each polymerase has a specific function and is recruited to a specific promoter sequence. RNA polymerase I (Pol I) is responsible for the syn thesis of only one type of RNA, a transcript called pre ribosomal RNA (or pre-rRNA), which contains the pre cursor for the 18S, 5.8S, and 28S rRNAs . Pol I promoters vary greatly in sequence from one species to another. The principal function of RNA polymerase II (Pol II) is synthesis of mRNAs and some specialized RNAs. This enzyme can recognize thousands of promoters that vary greatly in sequence. Many Pol II promoters have a few sequence features in common, in cluding a TATA box (eukaryotic consensus sequence TATAAA) near base pair -30 and an Inr sequence (initiator) near the RNA start site at +1 (Fig. 1). RNA polymerase III (Pol III) makes tRNAs, the 5S rRNA, and some other small specialized RNAs. The promoters recognized by Pol III are well characterized. Interestingly, some of the sequences required for the reg ulated initiation of transcription by Pol III are located within the gene itself, whereas others are in more conventional locations upstream of the RNA start site.
FIGURE 1 Common sequences in promoters recognized by eu karyotic RNA polymerase II. The TATA box is the major assembly point for the proteins of the preinitiation complexes of Pol II. The DNA is unwound at the initiator sequence (Inr), and the transcription start site is usually within or very near this sequence. In the Inr consensus sequence shown here, N represents any nucleotide; Y, a pyrimidine nu cleotide. Many additional sequences serve as binding sites for a wide variety of proteins that affect the activity of Pol II. These sequences are important in regulating Pol II promoters and vary greatly in type and number, and in general the eukaryotic promoter is much more com plex than suggested here. Many of the sequences are located within a few hundred base pairs of the TATA box on the 5 side; others may be thousands of base pairs away. The sequence elements summarized here are more variable among the Pol II promoters of eukaryotes than among the E. coli promoters (see Fig. 26–5). Many Pol II promoters lack a TATA box or a consensus Inr element or both. Additional sequences around the TATA box and downstream (to the right as drawn) of Inr may be recognized by one or more transcription factors.
