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Mouse kidney nonpolysomal messenger ribonucleic acid: metabolism, coding function, and translational activity
Abstract:
To elucidate the distribution and function of mRNA in mouse kidney cytoplasm, we compared mRNA isolated from polysomal (greater than 80S) and native postpolysomal (20--80S) ribonucleoproteins with respect to synthesis and lifetime, sequence content, and translational activity. The 20--25% of cytoplasmic mRNA recovered from postpolysomal ribonucleoprotein is similar to polysomal mRNA in size (20--22S), in apparent half-life (11--13 h), in major products of cell-free translation, and in nucleotide complexity (approximately 4 x 10(7) nucleotides). The labeling kinetics of polysomal and postpolysomal mRNA suggest these mRNA populations are in equilibrium. [3H]cDNAs transcribed from polysomal and from postpolysomal poly(A)-containing mRNAs react with template mRNA and with the heterologous mRNA at the same rate (Cot1/2 approximately 6.3 mol.s/L) and to the same extent (95%). Therefore, these mRNAs are equally diverse and homologous and occur at similar relative frequencies. Postpolysomal mRNA directs cell-free protein synthesis at only approximately 30% of the rate of polysomal mRNA and to only 30% of the extent of mRNA from polysomes. Postpolysomal mRNA is approximately 3-fold less sensitive than polysomal mRNA to inhibition of translation by m7GMP, suggesting postpolysomal mRNA contains a greater fraction of molecules deficient in 5'-terminal caps. Postpolysomal mRNA may derive from renal mRNAs that initiate translation inefficiently and thus accumulate as postpolysomal ribonucleoproteins.
Insights
Mouse kidney mRNA exists in both active polysomal and less active postpolysomal forms. Postpolysomal mRNA, though similar in sequence, shows reduced translational efficiency, possibly due to cap deficiency.
Area of Science:
- Molecular Biology
- Cell Biology
- Genetics
Background:
- Messenger RNA (mRNA) localization and activity within the cytoplasm are crucial for gene expression.
- Understanding the distinct functional states of mRNA, such as polysomal and postpolysomal populations, provides insights into translational regulation.
Purpose of the Study:
- To investigate the distribution and functional characteristics of mRNA in mouse kidney cytoplasm.
- To compare mRNA from polysomal and postpolysomal ribonucleoproteins regarding synthesis, lifetime, sequence content, and translational activity.
Main Methods:
- Isolation and comparison of mRNA from polysomal (>80S) and postpolysomal (20-80S) ribonucleoproteins.
- Analysis of mRNA size, half-life, cell-free translation products, and nucleotide complexity.
- Kinetic analysis of mRNA labeling and cDNA hybridization (Cot1/2) to assess diversity and homology.
- Assessment of translational inhibition by m7GMP to evaluate 5'-terminal cap status.
Main Results:
- Postpolysomal mRNA (20-25% of cytoplasmic mRNA) shares similarities with polysomal mRNA in size, half-life, and nucleotide complexity.
- Both mRNA populations are equally diverse, homologous, and present at similar relative frequencies.
- Postpolysomal mRNA exhibits significantly lower translational activity (approx. 30%) and is less sensitive to m7GMP inhibition, suggesting a higher proportion of uncapped molecules.
Conclusions:
- Mouse kidney mRNA populations in polysomal and postpolysomal fractions are in equilibrium.
- Postpolysomal mRNA may represent molecules with inefficient translation initiation, potentially due to 5'-terminal cap deficiencies, leading to their accumulation.