| Summary: | RNA Polymerase II (Pol II) transcribes mRNA genes, snRNA genes and histone genes. Whereas mRNA genes often contain introns, snRNA genes are intron-Iess. mRNA genes have a polyadenylation signal and snRNA genes contain a gene-specific 3' end processing signal, the 3' box. The length of a U2 snRNA transcription unit is approximately 1 kB, whereas that ofa mRNA gene is often thousands ofbase pairs long. A snRNA promoter is required for efficient 3' box processing. I have confIrmed these results but my data also show that a snRNA promoter can direct synthesis of poly (A) tails efficiently. It has been demonstrated that 3' box processing requires the conserved carboxyl tenninal domain (CTD) of pol II. It has been shown that the phosphorylation of the CTD on serine 2 by P-TEFb is essential for efficient 3' box processing. P-TEFb is also an elongation factor in the transcription ofmRNA genes. Two elements that have been reported to recmit P-TEFb were introduced into the U2 and the H2b gene: the HIV-I TARffat complex and an intron. I have shown that the HIV-I TARffat complex introduced into a U2 construct interferes with 3' box processing and . increases the size of the transcription unit. However, the TARfTat complex is compatible with H2b 3' end processing. These effects suggest a possible change of P-TEFb function through HIV-I TARfTat from snRNA 3' box processing to mRNA elongation factor. A synthetic PY7 intron was introduced into both genes. It is spliced efficiently. No disabling of the promoter-specific 3' end is detectable in either gene. The site of transcription termination is unaffected by the PY7 intron. It was concluded that the characteristic lack of introns in histone and snRNA genes is not a requirement for efficient, gene-specific 3' end processing. Substitution of the CMV promoter for a snRNA promoter has a negative effect on 3' box processing as well as an increase in the size of the transcription unit, emphasizing the importance of the promoter in defining the method of 3' end processing and the size of the transcription unit.
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