Differential usage of iso-accepting tRNASer species in silk glands of Bombyx mori

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Abstract
The rapid development of the silk glands of B. mori during the last larval instar shows 2 phases. During the 1st 4 days, in both the middle and posterior parts of the silk glands, the ribosomal machinery is assembled and the synthesis of house-keeping proteins starts. During the 2nd phase (the last 4 days), the middle part of the gland synthesizes .apprx. 45 mg of the silk protein sericin (31% serine) and the posterior part of the gland synthesizes .apprx. 130 mg of the silk protein fibroin (46% glycine, 29% alanine and 12% serine). Silk fibroin and sericin are detectable by the 2nd day and represent 80 and 50% respectively of the total proteins produced at day 8. The tRNA population of the posterior part of the gland is quantitatively adapted to fibroin codon frequency during this period but little is known about the situation in the middle part except for the observation that it contains more tRNASer than does the posterior part. The 2 parts were shown here to contain, and presumably use, different iso-accepting species of tRNASer, the middle part using tRNA1Ser, which recognizes AGU and AGC codons, and the posterior part using tRNA2Ser which recognizes UCA. This differential adaptation of the tRNASer species apparently is under transcriptional control as the 2 spp. are accumulated at different rates, but degraded at the same rate.
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