Complete structures of three rice sucrose synthase isogenes and differential regulation of their expressions.

By cloning and sequencing cDNA and genomic DNA and transcription initiation site mapping, the total structures including at least 1 kb of putative regulatory sequences upstream of the transcription initiation sites of three genes encoding rice sucrose synthase isoprotomers were either newly established or amended. The third type of SS gene, RSus3, has not been found in other plants. The structures of the three genes and the gene products were compared and their evolutionary sequence was proposed. Specific probes for the three SS mRNA's were developed and used for analyzing their steady state levels at different organs and under some physiological stress conditions. It appears that RSus2 is a house-keeping gene, RSus3 is highly specific to the grain, and the expression of RSus1 shows a tendency to complement that of RSus3. A possible cause of the presence of the third rice SS gene was discussed. We also reported a novel method to synthesize single-stranded DNA for S1 mapping of a transcription initiation site associated with extended secondary structures.