Localization of RNA polymerase in polytene chromosomes of Drosophila melanogaster.

RNA polymerase (RNA nucleotidyltransferase) B (or II) and histone H1 of Drosophila melanogster were localized on salivary gland polytene chromosomes using the indirect immunofluorescence technique. RNA polymerase B is present almost exclusively in puffs and interband regions, whereas histone H1 is found primarily in bands. The puff at region 3C, known to be transcriptionally active in larval salivary glands, gives a bright fluorescence with antibodies against RNA polymerase B. This fluorescence disappears after exposure of the larvae to 37 degrees for 45 min. The heat shock treatment results in a general reduction of fluorescence intensity with the appearance of brightly staining heat shock puffs. Heat-induced removal of RNA polymerase molecules from a puff does not immediately alter its morphology. We propose than an interband represents that fraction of the total number of gene copies in a band that are active, the inactive copies being present in a condensed form in the adjacent band. Large puffs would originate through the decondensation and activation of most or all gene copies in a band.