HeLa cell poly(A)- mRNA codes for a subset of poly(A)+ mRNA-directed proteins with an actin as a major product

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Abstract
Poly(A)+ and poly(A)- mRNA from HeLa cells were separated and translated in heterologous messenger-dependent protein synthesizing systems. Two-dimensional electrophoretic analysis revealed three classes of polypeptides. At the level of detectability in the electropherograms, a small number (about 10) of proteins were detected only among the poly(A)- mRNA products, a larger number (about 40) were produced by both poly(A)- and poly(A)+ mRNA, and a large number of polypeptides were found exclusively in the poly(A)+ mRNA products. The major product of both poly(A)+ and poly(A)- mRNA was shown to be the β form of actin. Previous cross hybridization measurements suggested little homology between poly(A)+ and poly(A)- mRNA populations. In view of the apparent identity of many poly(A)- products with those of poly(A)+, the homology between poly(A)+ and poly(A)- mRNA sequences was examined in greater detail. cDNA complementary to only the most abundant poly(A)+ message sequences was prepared. About 10% of this cDNA hybridized to abundant sequences in the poly(A)- fraction. This corresponded to only 2% of the total mass of poly(A)+ mRNA and accounted for the failure to detect cross hybridization in previous experiments. Thus, a small number of poly(A)+ sequences appear to be present in relatively high concentration in poly(A)- mRNA as evidenced by both the translation products and the cross hybridization results.