The orotate transporter oroP from Lactococcus lactis is required for orotate uptake and can be used both as a food-grade selection and counter-selection marker

The plasmid pDBORO conferred sensitivity of Lactococcus lactis towards 5´-Fluoroorotate. Moreover, by transformation, this plasmid was found to be essential for orotate dependent growth in a pyrimidine requiring strain. A single gene on pDBORO was required for growth on orotate, and was termed oroP.

Since the corresponding open reading frame, OroP showed the features of a membrane protein, OroP most likely is an orotate transporter. A number of strains like L. lactis and Bacillus subtilus have been shown to be unable to metabolize orotate. If the oroP gene was introduced into these strains, they acquired the ability to utilize orotate.

If the strains had a pyrimidine requirement, the oroP gene could function as a selectable marker when growing in the presence of orotate as sole pyrimidine source. In an otherwise resistant strain, oroP was shown to sensitize the strain towards the analog 5-Fluoroorotate. Strains that have lost the oroP gene could easily be selected in the presence of 5-Fluoroorotate, thus being an efficient counter-selection marker.

In addition to L. lactis and B. subtilus the system has been proven efficient in other strains including Escherichia coli and Bacillus licheniformis.