SYNECHOCYSTIS PCC-6803 CONTAINS A SINGLE-GENE FOR THE BETA-SUBUNIT OFTRYPTOPHAN SYNTHASE WITH STRONG HOMOLOGY TO THE TRPB GENES OF ARABIDOPSIS AND MAIZE (ZEA-MAYS L)

We report the sequence of the trpB gene of the cyanobacterium Synechoc ystis sp. PCC 6803. This gene was cloned from a plasmid library by fun ctional complementation of a trpB mutant of Escherichia coli K-12. Amo ng the known trpB sequences, the Synechocystis gene bears the greatest homology to the duplicated trpB genes of Arabidopsis thaliana and Zea mays. Southern and northern blotting analyses suggest that Synechocys tis contains only a single trpB gene. In contrast to all other prokary otes, Synechocystis has a trpB gene that is monocistronic. Attempts to construct a trpB null mutant of Synechocystis by standard techniques were unsuccessful, suggesting that this organism is unable to concentr ate tryptophan from the external medium.