Likely catalyzes the epimerization of trans-4-hydroxy-L-proline (t4LHyp) to cis-4-hydroxy-D-proline (c4DHyp). May be involved in the degradation pathway that converts t4LHyp to alpha-ketoglutarate, which would allow R.meliloti to grow on t4LHyp as a sole carbon source. trans-4-hydroxy-L-proline = cis-4-hydroxy-D-proline Is slightly up-regulated when the bacterium is grown on t4LHyp or t3LHyp as sole carbon source. Belongs to the proline racemase family.