Expression and hormonal control of a new larval cuticular multigene family at the onset of metamorphosis of the tobacco hornworm.

The pattern of cuticular protein synthesis by the epidermis of the tobacco hornworm larva changes during the final day of feeding, leading to an alteration in cuticular structure and a stiffening of the cuticle. We have isolated a small multigene family which codes for at least three of the new cuticular proteins made at this time. The five genes which were isolated from this family map to two different genomic regions. Sequencing shows that one of the genes is 1.9 kb and consists of three exons coding for a 12.2-kDa acidic (pI = 5.26) protein that is predominantly hydrophilic. The deduced amino acid sequence shows regions of similarity to proteins from flexible lepidopteran cuticles and from Drosophila larval and pupal cuticles, but not to proteins found in highly sclerotized cuticles. This gene family is first expressed late on the penultimate day (Day 2) of feeding in the final larval instar and ceases expression 2 days later when metamorphosis begins. In situ hybridization shows that this gene family is expressed in all the epidermal cells of Day 3 larvae except the bristle cells and those at the muscle attachment site. Expression can be induced in Day 1 epidermis by exposure to 50 ng/ml 20-hydroxyecdysone in vitro, but only if juvenile hormone is absent. Its developmental expression, tissue specificity, and hormonal regulation strongly suggest that this multigene family is involved in the structural changes that occur in the larval cuticle just prior to the onset of metamorphosis.