Heat shock responses in polytene foot pad cells of Sarcophaga bullata

Heat shock induces a single large puff (hs puff) near the tip of chromosome arm EL in polytene foot pad cells of fly pupae (Sarcophaga bullata). The inducible hs locus is constitutively active, invariably forming a small puff, which can be maximally activated in cells of the dorsal epidermis or in trichogen cells at any time during the lifetime of mature polytene chromosomes. Both in vivo and in cultured food pads, maximal puff induction occurs at 37 degrees C. At the same temperature, normal development of puffing patterns continues undisrupted for several days. A few specific hs proteins are vigorously induced at 37 degrees C, also without disrupting patterns of normal protein synthesis. Rates of normal protein synthesis in cultured food pads and rates of pupal development are enhanced up to about 39 degrees C. During heat shock at 41 degrees-44 degrees C protein synthesis becomes completely dominated by the production of hs proteins. The severe or complete suppression of most of the proteins normally made is followed by developmental arrest. There is also a decline of transcription (chromosomal uridine incorporation) between 37 degrees and 44 degrees C, which appears to affect all chromosomal loci proportionally, including the hs locus. The hs puff is no longer maximally induced at 41 degrees-44 degrees C, but the expanded puff now persists indefinitely, whereas below 39 degrees C, initial puff expansion is always followed by at least partial puff regression. The control of the duration of the puffing response appears to be entirely independent of protein synthesis, e.g., complete inhibition of protein synthesis by cycloheximide fails to prolong transient puffing responses. Canavanine also has no effect on puff regression. Heat shock above 45 degrees C arrests all RNA and protein synthesis within 30 min. RNA synthesis is resumed immediately after shift-down to 25 degrees C, not only at the hs locus, but at most or all previously active loci. Protein synthesis is also resumed immediately, but it is almost completely restricted to the production of the major hs protein (hsp-65, equivalent to hsp-70 of Drosophila melanogaster). Extreme heat shock also triggers maximal puffing responses at the hs locus, but actual puff expansion is delayed and only occurs hours after shift-down in the wake of a surge of hsp-65 synthesis. Following these delayed hs responses pupal thermotolerance starts increasing and protein synthesis returns to normal.