Chloroplast Replication : Evidence for 5-Bromouracil Incorporation and Plastid Mutation in Euglena gracilis

Abstract

THERE is now growing experimental support for the hypothesis that nucleoprotein genetic determinants located outside the nucleus confer photosynthetic activity on plastids. The first clear indication of nucleoprotein involvement in plastid mutation was derived from the action spectrum for ultra-violet inactivation of Euglena plastids which showed peaks at 260 and 280 mµ¹. This ultra-violet sensitive plastid determinant is photoreactivable², and evidence for its cytoplasmic nature has been reported^1–3. Comparison of wild-type and ultra-violet-induced mutants of Euglena strongly suggests that a DNA of different base composition from the nucleus is uniquely associated with the ability to form plastids⁴. Similar findings have been reported for plastids of other algae^4–8 and higher plants^6,9. The recent observation¹⁰ that cells which incorporate azathymine in place of thymine exhibit increased resistance to ultra-violet inactivation of plastids is consistent with the reports that plastids have associated with them a specific DNA, and serves as a guide to the rational search for chemical agents that might be expected to induce plastid mutations. By analogy with other nucleoprotein genetic systems, we have postulated that incorporation of base analogues into the genetic material that determines plastids should lead to mutational changes, and that such changes would be reflected in altered plastid structure and function. This communication presents evidence for incorporation of 5-bromouracil into cells of Euglena gracilis, and describes conditions for obtaining stable colourless plastid mutants during replication of plastid determinants in the presence of 5-bromouracil and sulphanilamide.