Differential usage of iso-accepting tRNA^Ser species in silk glands of Bombyx mori

Abstract

The rapid development of the silk glands of Bombyx mori during the last larval instar shows two phases. During the first 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 second phase (the last 4 days), the middle part of the gland synthesizes ∼45 mg of the silk protein sericin (31% serine) and the posterior part of the gland synthesizes ∼130 mg of the silk protein fibroin (46% glycine, 29% alanine and 12% serine). Silk fibroin and sericin are detectable by the second day and represent 80 and 50% respectively of the total proteins produced at day 8 (refs 1–4). It is known that the tRNA population of the posterior part of the gland is quantitatively adapted to fibroin codon frequency during this period^5–9 but little is known about the situation in the middle part except for the observation that it contains more tRNA^Ser than does the posterior part. We show here that the two parts contain, and presumably use, different iso-accepting species of tRNA^Ser, the middle part using tRNA₁^Ser, which recognizes AGU and AGC codons, and the posterior part using tRNA₂^Ser which recognizes UCA. We also suggest that this differential adaptation of the tRNA^Ser species is under transcriptional control as the two species are accumulated at different rates, but degraded at the same rate.