Synechocystis PCC 6803 contains a single gene for the beta subunit of tryptophan 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 Synechocystis sp. PCC 6803. This gene was cloned from a plasmid library by functional complementation of a trpB mutant of Escherichia coli K-12. Among 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 Synechocystis contains only a single trpB gene. In contrast to all other prokaryotes, 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 concentrate tryptophan from the external medium.