Tritiated-histidine incorporation into Drosophila salivary chromosomes

An autoradiographic study of H3‐histidine incorporation into nonhistone protein of explanted larval salivary gland chromosomes of D. virilis showed patterns of incorporation that were dependent upon the stage of larval development. The sequence of changes in the development of several puffs in a specific chromosomal region was followed using the appearance of pigment in the anterior spiracles as a means of larval staging. H3‐histidine incorporation into these puffs in prepupae occurred as the puffs were regressing in size and protein staining. Acid extraction of histone and nucleic acid failed to alter the character of the autographs; presumably a non‐histone protein is involved in the H3‐histidine incorporation. Other puff sites in the same prepupal chromosomes showed various patterns of isotopic amino acid incorporation indicating that the pattern reported for a specific region may not be true for all puff sites.