Maintenance of the DNA puff expanded state is independent of active replication and transcription

The maintenance of the expanded state of DNA puffs II/2B and II/9A in polyt ene chromosomes from stage 14x7 Sciara coprophila salivary glands was assay ed after inhibition of RNA synthesis, DNA synthesis, or both processes toge ther. Heat shock conditions were established in order to inhibit transcript ion. Polypeptides of M-r 72,000 and 36,000 were produced in Sciara after he at shock. The gene encoding the M-r 72,000 polypeptide, the homolog of Dros ophila hsp70, was cloned. In situ hybridization detected Sciara hsp70 at ba nds 4A and 17C on chromosome IV. Sciara hsp70 encodes a 2.3 kb heat shock m RNA. DNA puffs (e.g., DNA puffs 2B and 9A on chromosome II) remained fully expanded even after inhibition of transcription by heat shock or actinomyci n D, or after inhibition of DNA replication by aphidicolin, or inhibition o f both RNA synthesis and DNA synthesis together by actinomycin D plus aphid icolin. Therefore, maintenance of the DNA puff expanded state in Sciara doe s not require ongoing transcription and/or replication. Mechanisms for init iation and for maintenance of puffs (open chromatin structure) are discusse d.