As a component of the mitochondrial large ribosomal subunit, plays a role in mitochondrial translation (PubMed:23603806). When present in mitochondria as a free protein not associated with the ribosome, associates with mitochondrial RNA polymerase POLRMT to activate transcription (PubMed:22003127). Required for POLRMT stability (PubMed:26586915). {ECO:0000269|PubMed:22003127, ECO:0000269|PubMed:23603806, ECO:0000269|PubMed:26586915}. This is the function of Ensembl gene identifier ENSG00000262814 (MRPL12, mitochondrial ribosomal protein L12).