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Structure and function of rat liver polysome populations. II. Characterization of polyadenylate-containing mRNA
Abstract:
Poly(A)+RNA fractions prepared from free and loosely and tightly membrane-bound polysome populations (poly(A)+RNAfree, poly(A)+RNAloose, and poly(A)+RNAtight) were used to drive cDNA in homologous and heterologous hybridization reactions. A large fraction by mass of sequences was shared among the three poly(A)+RNA populations, but shared sequences exhibited distinct frequency distributions within the different populations. 13-15 in vitro translation products of poly(A)+RNAfree and poly(A)+RNAloose detected by gel electrophoresis were shared. Most of these were produced in different relative quantities by the two RNA populations. Five or six higher mol wt polypeptides were produced by poly(A)+RNAloose that were not detected as products of either poly(A)+free or poly(A)+RNAtight. We suggest that loosely bound polysomes may not be artifactually derived as reflected in their quantitatively distinct poly(A)+RNA population. Two tightly membrane-bound RNP fractions were prepared from rat liver on the basis of their release from or retention on purified rough microsomes or a crude membrane fraction after in vitro disaggregation of polysomes with high-salt and puromycin. Homologous and heterologous hybridizations involving their poly(A)+RNA fractions revealed that a large portion by mass of sequences was shared but that these sequences exhibited distinct frequency distributions in the two fractions. The RNA fractions produced exhibited distinct frequency distributions in the two fractions. The RNA fractions produced an identical set of in vitro translation products but individual polypeptides were produced in different relative quantities. This indicates that the two RNP fractions do not arise by any random artifactual process and suggests that they may represent functionally distinct populations.
Insights
Investigating polysome-bound poly(A)+RNA in rat liver revealed distinct RNA populations. These populations, associated with free, loosely, and tightly bound polysomes, show quantitative differences in gene expression, suggesting distinct functional roles.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Polysomes, complexes of messenger RNA and ribosomes, are central to protein synthesis.
- The localization of polysomes (free vs. membrane-bound) is linked to the destination of synthesized proteins.
- Understanding the specific RNA content of different polysome populations is crucial for deciphering gene expression regulation.
Purpose of the Study:
- To characterize and compare the poly(A)+RNA populations from free, loosely bound, and tightly bound polysomes in rat liver.
- To investigate whether these distinct polysome populations represent artifactual or functionally differentiated entities.
- To analyze the in vitro translation products derived from these different RNA populations.
Main Methods:
- Isolation of poly(A)+RNA from free, loosely bound, and tightly bound polysome fractions.
- cDNA synthesis and subsequent homologous and heterologous hybridization reactions to compare RNA sequences.
- In vitro translation of isolated RNA followed by gel electrophoresis to analyze polypeptide products.
Main Results:
- A significant overlap in RNA sequences was observed across all three polysome populations, but with distinct frequency distributions.
- In vitro translation revealed shared and unique polypeptides, with differing relative quantities produced by free and loosely bound polysomes.
- Two distinct tightly membrane-bound RNP fractions yielded identical sets of translation products but in different relative amounts, indicating non-random origins.
Conclusions:
- The quantitative differences in poly(A)+RNA and translation products suggest that loosely bound polysomes are not artifactual.
- The distinct RNA profiles and translation products of membrane-bound RNP fractions indicate they represent functionally specialized populations.
- These findings support the hypothesis that polysome localization is linked to specific functional roles in protein synthesis and regulation.