TmGUSI, a gene identical to that encoding a thermostable beta-glucuronidase in the hyperthermophilic anaerobe Thermotoga maritima, has been synthesized using a PCR-based two-step DNA synthesis and codon optimization for plants, and expressed in both Escherichia coli and Arabidopsis thaliana. TmGUSI expressed in transformed E. coli cells exhibited maximum hydrolytic activity at 65 A degrees C and pH 6.5 and retained more than 80% activity after incubation at 85 A degrees C for 30 min. TmGUSI activity in transgenic A. thaliana plants containing TmGUSI was also stable over the temperature range 65-80 A degrees C. Our data suggest that beta-glucuronidase from T. maritima can serve as a useful thermostable marker in higher plants.