RNA N6-methyltransferase that methylates adenosine residues at the N(6) position of a subset of RNAs and is involved in S-adenosyl-L-methionine homeostasis by regulating expression of MAT2A transcripts. Able to N6-methylate a subset of mRNAs and U6 small nuclear RNAs (U6 snRNAs). In contrast to the METTL3-METTL14 heterodimer, only able to methylate a limited number of RNAs: requires both a 5'UACAGAGAA-3' nonamer sequence and a specific RNA structure. Plays a key role in S-adenosyl-L-methionine homeostasis by mediating N6-methylation of MAT2A mRNAs, altering splicing of MAT2A transcripts: in presence of S-adenosyl-L-methionine, binds the 3'-UTR region of MAT2A mRNA and specifically N6-methylates the first hairpin of MAT2A mRNA, impairing MAT2A splicing and protein expression. In S-adenosyl-L-methionine-limiting conditions, binds the 3'-UTR region of MAT2A mRNA but stalls due to the lack of a methyl donor, preventing N6-methylation and promoting expression of MAT2A. In addition to mRNAs, also able to mediate N6-methylation of U6 small nuclear RNA (U6 snRNA): specifically N6-methylates adenine in position 43 of U6 snRNAs. adenosine in U6 snRNA + S-adenosyl-L-methionine = H(+) + N(6)-methyladenosine in U6 snRNA + S-adenosyl-L-homocysteine an adenosine in mRNA + S-adenosyl-L-methionine = an N(6)-methyladenosine in mRNA + H(+) + S-adenosyl-L-homocysteine Methyltransferase activity is autoinhibited by the K-loop region that blocks S-adenosyl-L-methionine-binding. Upon activation, K-loop changes conformation, allowing S-adenosyl-L-methionine-binding and subsequent methyltransferase activity. mRNA N6-adenosine-methyltransferase activity is inhibited by zinc. The VCR (vertebrate conserved) regions bind the first hairpin of MAT2A mRNAs. The VCR regions interact with the internal stem-loop within U6 snRNAs, inducing the conformational rearrangement of the A43-containing region of U6 snRNA, thereby modifying the RNA structure to become suitable for productive catalysis by the methyltransferase region. The K-loop region occludes the S-adenosyl-L-methionine-binding pocket. Upon activation, conformation of the K-loop changes, allowing S-adenosyl-L-methionine-binding. Belongs to the methyltransferase superfamily. METTL16/RlmF family.