Activation of potential initiation sites of DNA replication and induction of new initiation within the same cycle by puromycin in Drosophila polytene chromosomes.

DNA replication in the polytene chromosomes of larval salivary glands of Drosophila hydei has been examined under normal physiological conditions, with or without puromycin, by an autoradiographic procedure using [3H]thymidine. It has been demonstrated that puromycin induces new labelling patterns that can be identified as initiation patterns. Such initiation patterns were found to have been induced during both the initial (interband labelling patterns) and the terminal (discontinuous) phases. Both induced patterns are sensitive to alpha-amanitin. These findings lead us to suggest that (a) the puff-interband-labelled patterns are indeed the initial patterns, and (b) the induction of new initiation, which is normally known to be prevented until the completion of the cycle, can be brought about by puromycin. The results also suggest the probable existence of a pool of protein factor(s) for the initiation of a replication cycle already present in the cell system.