Differential Synthesis Rates of tRNA Species in the Silk Gland of Bombyx mori Are Required to Promote tRNA Adaptation to Silk Messages

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Abstract
The accumulation rates of tRNA species rapidly labeled in vivo were investigated in the posterior and middle silk glands of the silkworm B. mori during the last larval instar. The rates of synthesis of total and individual tRNA species labeled with [3H]uridine during the growth and the secretion phases and separated by electrophoresis on polyacrylamide gels, are specific for each tRNA species and each tissue. Continuous variations of the tRNA levels designed to reflect changes in the population of mRNA were noted. The tRNA adaptation to silk mRNA is completed roughly at the end of the growth phase and at the beginning of the secretion phase (day 4 of the Vth instar). Its stabilization during the secretion phase occurs by a reversal of the relative rates of predominant tRNA species, mainly tRNAGly. The sum total of the synthetic rates for each tRNA species corroborates their known adaptive distribution during the secretion phase of silk proteins. When coupled to the similarity of their turnover rates, observed previously, a transcriptional control of tRNA genes is supported.