The mtaCB1, mtaCB2 and mtaCB3 operons encode isozymes of methanol methyltransferases in Methanosarcina acetivorans C2A and are among the most highly regulated genes known in Archaea. Here we identify cis and trans acting elements that affect the expression of these operons. In vivo reporter gene constructs expressed from sequentially truncated promoters show that the mRNA transcripts for these operons have large 5' untranslated regions. Regions upstream of the transcription start site (TSS) are important for induction of the mtaCB1 and mtaCB2 operons and for repression of the mtaCB3. Regions downstream of the TSS are important for expression of the mtaCB2 and mtaCB3 operons, but are dispensable for mtaCB1 expression. Proteins encoded by the MA0459 and MA0460 loci are required for induction of the mtaCB1 operon, while those encoded by MA4383, MA4397 and MA4398 are required for induction of mtaCB2. Proteins encoded by MA4397 and MA4398 are also required for methanol-specific repression of the mtaCB3 operon and, thus, are the first archaeal examples of regulatory proteins that simultaneously act in both repression and activation. We refer to these genes as methanol-specific regulators and designate MA0459, MA0460, MA4383, MA4397 and MA4398 as msrA, msrB, msrC, msrD and msrE respectively.