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Four factors are required for 3'-end cleavage of pre-mRNAs
Y Takagaki1, L C Ryner, J L Manley
1Department of Biological Sciences, Columbia University, New York, New York 10027.
Abstract:
We reported previously that authentic polyadenylation of pre-mRNAs in vitro requires at least two factors: a cleavage/specificity factor (CSF) and a fraction containing nonspecific poly(A) polymerase activity. To study the molecular mechanisms underlying 3' cleavage of pre-mRNAs, we fractionated CSF further and show that it consists of four separable subunits. One of these, called specificity factor (SF; Mr, approximately 290,000), is required for both specific cleavage and for specific polyadenylation and thus appears responsible for the specificity of the reaction. Although SF has not been purified to homogeneity, several lines of evidence suggest that it may not contain an essential RNA component. Two other factors, designated cleavage factors I (CFI; Mr, approximately 130,000) and II (CFII; Mr, approximately 110,000), are sufficient to reconstitute accurate cleavage when mixed with SF. A fourth factor, termed cleavage stimulation factor (CstF; Mr, approximately 200,000), enhances cleavage efficiency significantly when added to a mixture of the three other factors. CFI, CFII, and CstF do not contain RNA components, nor do they affect specific polyadenylation in the absence of cleavage. Although these four factors are necessary and sufficient to reconstitute efficient cleavage of one pre-RNA tested, poly(A) polymerase is also required to cleave several others. A model suggesting how these factors interact with the pre-mRNA and with each other is discussed.
Insights
Researchers identified four key protein factors involved in pre-mRNA 3' cleavage and polyadenylation. Specificity factor (SF) is crucial for accurate cleavage and polyadenylation, working with other factors like cleavage factors I and II, and cleavage stimulation factor (CstF).
Area of Science:
- Molecular Biology
- RNA Processing
- Gene Expression Regulation
Background:
- Authentic polyadenylation of pre-mRNAs in vitro requires cleavage/specificity factor (CSF) and poly(A) polymerase.
- Understanding the molecular mechanisms of 3' pre-mRNA cleavage is essential for elucidating gene expression regulation.
Purpose of the Study:
- To further fractionate and characterize the components of the cleavage/specificity factor (CSF).
- To identify the specific roles of individual subunits in pre-mRNA 3' cleavage and polyadenylation.
Main Methods:
- Biochemical fractionation of the cleavage/specificity factor (CSF).
- Reconstitution assays using purified subunits to test for cleavage and polyadenylation activity.
- Analysis of subunit composition and potential RNA components.
Main Results:
- CSF comprises four separable subunits: specificity factor (SF), cleavage factors I (CFI), II (CFII), and cleavage stimulation factor (CstF).
- SF is essential for specific cleavage and polyadenylation, indicating its role in reaction specificity.
- SF, CFI, and CFII reconstitute accurate cleavage; CstF enhances efficiency. None of these factors contain essential RNA components.
Conclusions:
- The four identified factors (SF, CFI, CFII, CstF) are necessary and sufficient for efficient pre-mRNA 3' cleavage.
- A model for the interaction of these factors in pre-mRNA processing is proposed.
- Poly(A) polymerase is also required for the cleavage of some pre-mRNAs, highlighting the complexity of the polyadenylation process.