Molecular characterization of a cDNA encoding extracellular dsRNase and its expression in the silkworm, Bombyx mori.

A double-stranded ribonuclease (Bm-dsRNase) was separated from the digestive juice of the silkworm larvae, Bombyx mori. The full-length cDNA was produced and sequenced using a 20 mer primer designed from the N-terminal sequence of the Bm-dsRNase. The cDNA had an ORF encoding 51 kDa precursor protein which can be divided into three domains: a signal peptide, an N-terminal propeptide and a mature Bm-dsRNase. The precursor has an Arg-Ser cleavage site, which produces the 43 kDa mature protein by post-translational processing. The 43 kDa protein had conserved catalytic amino acid residues which are also found in the active site of the Serratia marcescens dsRNase. Expression of the precursor occurred in the middle and posterior midgut tissues, starting from Day 1 of the fifth instar larvae. The 43 kDa protein was produced in this tissue from Day 2, and coincidentally secreted into the lumen containing digestive juice. This was supported by the immunohistochemical observation that the mature proteins were localized in the apical side of midgut cells for extracellular secretion.