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Alternative processing of mRNAs encoding mammalian chromosomal high-mobility-group proteins HMG-I and HMG-Y
K R Johnson1, D A Lehn, R Reeves
1Program in Genetics and Cell Biology Washington State University, Pullman 99164.
Abstract:
The high-mobility-group protein HMG-I is a well-characterized nonhistone chromosomal protein that is preferentially expressed in rapidly dividing cells, binds to A. T-rich regions of DNA in vitro, and has been localized to particular regions of mammalian metaphase chromosomes. We isolated eight cDNA clones encoding HMG-I and its isoform HMG-Y from a human Raji cell cDNA library and detected blocks of nucleotide sequence rearrangements in the 5'-untranslated regions of these clones. In addition to this leader sequence variation, five of the eight cDNA clones had either a 33- or 36-base-pair in-frame deletion in their open reading frame (ORF); we found that this shortened ORF encodes the HMG-Y protein isoform. We present evidence that the 5'-untranslated-region and ORF heterogeneity of the cDNA clones is the result of alternative processing of RNA transcripts from a single functional gene. Several additional but probably nonfunctional HMG-I or HMG-Y gene copies exist in the human genome; we isolated and partially sequenced one of these pseudogenes and found that it is a processed HMG-Y retropseudogene.
Insights
High-mobility-group protein HMG-I and its isoform HMG-Y arise from alternative RNA processing of a single gene. This explains sequence variations found in cDNA clones from human cells.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- High-mobility-group protein HMG-I is a nonhistone chromosomal protein.
- HMG-I is expressed in rapidly dividing cells and binds A.T-rich DNA regions.
- HMG-I localizes to specific regions of mammalian metaphase chromosomes.
Purpose of the Study:
- To investigate the molecular basis of HMG-I and HMG-Y isoforms.
- To analyze sequence variations in cDNA clones encoding HMG-I and HMG-Y.
- To determine the origin of HMG-I/HMG-Y heterogeneity.
Main Methods:
- Isolation of eight cDNA clones encoding HMG-I and HMG-Y from a human Raji cell library.
- Analysis of nucleotide sequence rearrangements in 5'-untranslated regions.
- Identification of in-frame deletions in the open reading frame (ORF).
- Sequencing of a human HMG-I/HMG-Y retropseudogene.
Main Results:
- Nucleotide sequence rearrangements were detected in the 5'-untranslated regions of cDNA clones.
- Five of eight cDNA clones exhibited 33- or 36-base-pair in-frame deletions in the ORF, encoding HMG-Y.
- Evidence suggests alternative RNA processing from a single functional gene generates HMG-I and HMG-Y isoforms.
- A processed HMG-Y retropseudogene was identified in the human genome.
Conclusions:
- HMG-I and HMG-Y protein isoforms result from alternative RNA processing of a single gene.
- Sequence heterogeneity in cDNA clones is attributed to alternative splicing and processing.
- The human genome contains multiple, likely nonfunctional, HMG-I/HMG-Y gene copies.