Direct correlation between a chromosome puff and the synthesis of a larval saliva protein in Drosophila melanogaster.

The structural gene Sgs-4 responsible for larval saliva protein 4 of Drosophila melanogaster was localized, with the help of Notch deficiencies, within the section between bands 3C10 and 3D1 of the X chromosome. In this chromosome section there is, very probably, only one fine band. In the third larval instar chromosome this section is transcriptionally active and forms a puff. When the ecdysone concentration increases, about 5 h before prepupa formation, it becomes inactive.--In section 3C of X chromosomes of third instar larvae of the stock Hikone-R no puff is formed. The saliva of these larvae lacks protein 4. However, female hybrids (H/B and H/O) from Hikone-R crossed with Berlin and Oregon respectively produce a Hikone-specific saliva protein 4h. The synthesis of protein 4h in the hybrids H/B and H/O is ascribed to an activation of the gene Sgs-4 in the Hikone chromosome.--In the saliva of heterozygotes (FM1/H) carrying one inversion chromosome In(1)FM1 and one X chromosome from Hikone, protein 4h could not be detected. In these inversion heterozygotes in 90% of all nuclei the homologues are not paired in 3C, and 3C is puffed only in the FM1 chromosome. This suggests that a precondition for the activation of Hikone gene Sgs-4 in heterozygotes may be intimate homologue pairing.--Intersexes with one of their X chromosomes from Hikone-R and the other from Berlin produce relatively more protein 4h than do diploid H/B females, indicating facilitated transcription as a result of dosage compensation.