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 (PubMed:29262316, PubMed:30197299). Able to N6-methylate a subset of mRNAs and U6 small nuclear RNAs (U6 snRNAs) (By similarity). 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 (By similarity). 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, preventing recognition of their 3'-splice site by U2AF1/U2AF35, thereby inhibiting splicing and protein production of S-adenosylmethionine synthase (By similarity). 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 (PubMed:29262316). 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 (By similarity). Also able to bind various lncRNAs, such as 7SK snRNA (7SK RNA) or 7SL RNA (By similarity). Specifically binds the 3'-end of the MALAT1 long non-coding RNA (By similarity) (PubMed:29262316, PubMed:30197299). 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. Interacts with MEPCE. Interacts with LARP7. 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. Embryonic lethality: embryos develop until blastocyst stage but development is stopped around the time of implantation (PubMed:30197299). Defects are caused by dysregulation of MAT2A mRNAs (PubMed:30197299). Belongs to the methyltransferase superfamily. METTL16/RlmF family. According to a report, N6-methylation of MAT2A affects MAT2A mRNA stability instead of preventing splicing (PubMed:29262316). However, it was later shown that N6-methylation of MAT2A transcripts prevents recognition of their 3'-splice site by U2AF1/U2AF35, thereby inhibiting splicing and protein production (By similarity).