Flimsy cocoon mutant of Bombyx mori larva produces a reduced amount of fibroin mRNA

The proportions of fibroin mRNA synthesized in the posterior silk gland of the flimsy cocoon mutant (flc) of Bombyx mori at days 3, 4 and 5 of the fifth instar were 1.2 %, 0.7 % and 1.8 % of the labeled total RNA, re-spectively, whereas those in heterozygous larva (+/flc) at the same stages were 1.6 %, 3.2 % and 8.5 %, respectively. The total amount of fibroin mRNA that accumulated in one larva at day 5 of flc was less than 10 μg and that of +/flc was about 110 μg. The intracellular distribution of the mRNA also was measured. The proportion in the nuclear fraction was 0.5 % of the total labeled RNA for flc and 0.8 % of that for +/flc; in the cytoplasmic fraction the pro-portion was 0.2 % for flc and 2.4 % for +/flc. Thus, the reduced amount of fibroin mRNA in the flc mutant is considered to be due to the degradation of the synthesized mRNA rather than to a reduction in the synthesis of mRNA. It is believed that this degradation causes abnormal fibroin production. The primary action site of the flc gene is not known.