Uncoupling of Chromatin Assembly from DNA Replication in Sciara Reveals a Domain of Postreplicative Immature Chromatin

DNA replication in dividing eukaryotic cells imposes a requirement for the faithful recreation on the newly synthesized chromatids of the nucleoprotein architecture of parent chromosomes. Practically nothing is known about the structure of postreplicative immature chromatin--a very short-lived entity (< 30 min.). We report here the unexpected discovery that during DNA amplification of locus II/9A in salivary gland polytene chromosomes of the fungus fly Sciara coprophila, DNA replication fork passage is uncoupled from postreplicative chromatin assembly; this enables visualization and analysis of chromatin fibers disassembled by DNA replication. We used electron microscopy to visualize a wealth of low nucleosome density immature chromatin fibers in preparations of Sciara chromatin from amplification-stage tissue. Remarkably, as gauged by high sensitivity to micrococcal nuclease and an unusually short length of DNA associated with each histone octamer, we found that locus II/9A which undergoes amplification and is replicated once every 4-6 hrs.--but not the bulk genome or a replicatively quiescent DNA stretch--was maintained in such an ummature fiber for ca. 24 hrs. Following amplification, locus II/9A assumed conventional chromatin organization, indicating that the epigenetic mark targeting nascent DNA to the chromatin assembly machinery is stable for several hours. We propose that this very unusual prolonged maintenance of a segment of the genome in immature chromatin facilitates access by the basal transcriptional machinery to the amplified DNA, and thus is an evolutionary adaptation to the demand for high transcription from genes that reside in the amplified loci.