The size of fibroin messenger RNA and its polyadenylic acid content

The molecular weight of fibroin messenger RNA has been measured by its contour length in the electron microscope and its electrophoretic mobility under conditions of complete denaturation. These methods give values of 5.6 times 10-6 and 6.0 times 10-6 daltons, respectively. The average molecular weight of 5.8 times 10-6 (1.6 times 10-4 base residues) would be expected to code for a protein of 414,000 daltons, a value about 12 percent larger than the most recently published size for fibroin. When the mRNA is prepared carefully to avoid shear breakage, it is as homogeneous in size as ribosomal RNA. Between 43 percent and 81 percent of the mRNA molecules bind to oligo(dT)-cellulose and are presumed to contain a poly(A) sequence. This sequence, isolated after ribonuclease digestion, comprises about 0.6 percent of the molecule, or an estimated 100 nucleotides. The poly(A) has been localized at the end of the molecule by hybridizing with a small poly (dT:dA) piece and subsequent visualization by electron microscopy. This observation, along with the ability of reverse transcriptase to transcribe fibroin mRNA with an oligo(dT) primer, demonstrates that the poly(A) is at the 3' end of the mRNA.