Mitochondrial tRNA N(1)-methyltransferase involved in mitochondrial tRNA maturation. Component of mitochondrial ribonuclease P, a complex composed of TRMT10C/MRPP1, HSD17B10/MRPP2 and PRORP/MRPP3, which cleaves tRNA molecules in their 5'-ends. Together with HSD17B10/MRPP2, forms a subcomplex of the mitochondrial ribonuclease P, named MRPP1-MRPP2 subcomplex, which displays functions that are independent of the ribonuclease P activity. The MRPP1-MRPP2 subcomplex catalyzes the formation of N(1)-methylguanine and N(1)-methyladenine at position 9 (m1G9 and m1A9, respectively) in tRNAs; TRMT10C/MRPP1 acting as the catalytic N(1)-methyltransferase subunit. The MRPP1-MRPP2 subcomplex also acts as a tRNA maturation platform: following 5'-end cleavage by the mitochondrial ribonuclease P complex, the MRPP1-MRPP2 subcomplex enhances the efficiency of 3'-processing catalyzed by ELAC2, retains the tRNA product after ELAC2 processing and presents the nascent tRNA to the mitochondrial CCA tRNA nucleotidyltransferase TRNT1 enzyme. In addition to tRNA N(1)-methyltransferase activity, TRMT10C/MRPP1 also acts as a mRNA N(1)-methyltransferase by mediating methylation of adenosine residues at the N(1) position of MT-ND5 mRNA. Associates with mitochondrial DNA complexes at the nucleoids to initiate RNA processing and ribosome assembly. adenosine(9) in tRNA + S-adenosyl-L-methionine = H(+) + N(1)-methyladenosine(9) in tRNA + S-adenosyl-L-homocysteine guanosine(9) in tRNA + S-adenosyl-L-methionine = H(+) + N(1)-methylguanosine(9) in tRNA + S-adenosyl-L-homocysteine an adenosine in mRNA + S-adenosyl-L-methionine = an N(1)-methyladenosine in mRNA + H(+) + S-adenosyl-L-homocysteine Component of mitochondrial ribonuclease P, a complex composed of TRMT10C/MRPP1, HSD17B10/MRPP2 and PRORP/MRPP3. Interacts with HSD17B10/MRPP2; forming the MRPP1-MRPP2 subcomplex of the mitochondrial ribonuclease P complex. Interacts with GRSF1. Belongs to the class IV-like SAM-binding methyltransferase superfamily. TRM10 family.