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RpiR-like protein. RpiR contains a SIS (Sugar ISomerase) domain, which is found in many phosphosugar isomerases and phosphosugar binding proteins. In E. coli, rpiR negatively regulates the expression of rpiB gene. Both rpiB and rpiA are ribose phosphate isomerases that catalyze the reversible reactions of ribose 5-phosphate into ribulose 5-phosphate.