YidC belongs to a family of membrane-localized chaperone-insertases present in bacteria, chloroplasts and mitochondria. It was discovered during study of the cariogenic bacterium Streptococcus mutans, that Gram-positive organisms encode two YidC paralogs. Identifying the respective functions of the dual paralogs is an important research goal. Elimination of Escherichia coli yidC is lethal. Elimination of S. mutans yidC1 compared to yidC2 has different phenotypic consequences, but both cannot be deleted simultaneously. S. mutans yidC1 and yidC2 reside at different loci under the control of separate promoters. The purpose of this work is to study the relative expression of yidC1 compared to yidC2 under various environmental conditions. To measure gene expression under the control of the yidC1 and yidC2 promoters, respectively, each promoter region was amplified by polymerase chain reaction using S. mutans genomic DNA as the template. We are now constructing ~700 bp promoter fusions in frame with the lacZ reporter gene in the pMZ plasmid. The engineered plasmids will next be transformed into S. mutans strain UA159, using competence stimulating peptide with selection on 1 mg/ml kanamycin. This will enable b-galactosidase assays of the engineered reporters strains to be performed under defined experimental conditions to reflect specific expression from the yidC1 compared to yidC2 gene promoter.