The blocked and methylated 5' terminal sequence of a specific cellular messenger: the mRNA for silk fibroin of Bombyx mori.

The messenger RNA for silf fibroin, laveled with 32PO4 and methyl-3H L-methionine, was purified to near homogeneity from the posterior silk gland of the sildworm Bombyx mori, and the sequence of a methylated, RNAase T2-resistant structure was determined. This sequence is similar structurally to 5' terminal blocked and methylated sequences found on the total populations of polyadenylated eucaryotic cellular and certain viral mRNAs. The RNAase T2-resistant oligomer from fibroin mRNA was cleaved by nucleases P1 into three components: a blocked and methylated sequence containing three phosphates; a 2'-0-methyl UMP residue (pUm), and an unmethylated CMP (pC). The blocked and methylated sequence comigrated in three chromatographic systems with the blocked and methylated terminus of silkworm cytoplasmic polyhedrosis virus mRNA, which has the structure m7GpppAm. The fibroin mRNA cap was cleaved by nucleotide pyrophosphatase to yeild 7-methyl GMP Tpm7G) and 2'-0-methyl AMP (pAm). This sequence also appeared to be terminally located, with the m7G joined by a 5'-5' pyrophosphate linkage to the Am. It was concluded that the 5' terminal sequence of fibroin mRNA molecules is m7G(5')ppp(5')AmpUmpCp. The regulation of expression of the highly specialized gene for fibroin is discussed in light of this finding.