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We have found that Chlamydomonas reinhardtii cells contain an ATP-dependent topoisomerase activity that supercoils circular DNA in vitro. Subsequent addition of a type I topoisomerase eliminates the supercoils. Like bacterial gyrase, this activity is inhibited by low concentrations of novobiocin (less than 0.1 microM) and by nalidixic acid (less than 0.1 microM). We have examined the effects of these topoisomerase inhibitors on accumulation of various chloroplast transcripts in vivo. Novobiocin differentially affected such transcripts; some transcripts became more abundant while many others were reduced in the presence of this drug. Nalidixic acid on the other hand caused many transcripts to become more abundant albeit to varying degrees. Inhibitors of this algal topoisomerase specifically stimulate a family of related transcripts which we have previously shown to be under light-dark control. We discuss how the inhibitors of this topoisomerase might exert their in vivo effects.

An ATP-dependent supercoiling topoisomerase ofChlamydomonas reinhardtiiaffects accumulation of specific chforoplast transcripts