Distribution of rat alpha-lactalbumin and casein mRNA between polyadenylated and nonpolyadenylated RNA.

mRNAs for the two types of milk protein were determined in the polyadenylated (poly(A)+) and nonpolyadenylated (poly(A)-) RNA fractions prepared from lactating rat mammary gland, to assess if a high fraction of poly(A)- mRNA is a general characteristic of milk protein mRNAs. Use of the [3H]poly(U) assay to evaluate the effectiveness of oligo(dT)-cellulose and poly(U)-Sepharose for the preparation of these samples showed that similar, essentially poly(A)-free, RNA samples could be prepared with either resin when the appropriate sample to resin ratio was used. The poly(A) tracts in rat mammary poly(A)+ RNA displayed bimodal distribution with peaks at 40 and 80 nucleotides. Translation of these samples confirmed previous observations that mammary poly(A)-RNA contains high amounts of translational activity; however, it is shown that translation gives an overestimate of the mRNA activity in this fraction. Analyses with complementary DNAs indicated that only 24-35% of the casein mRNA and 14-33% of the alpha-lactalbumin mRNA are present in poly(A)-RNA. Milk protein mRNAs are, therefore, comparable with other abundant mRNAs. Evidence is presented indicating that partial degradation can greatly reduce the extent of polyadenylation with little effect on the amount and on the translational activity of the mRNA.