The regulation of the Methanosarcina acetivorans mtsD, mtsF and mtsH genes, which encode putative corrinoid/methyltransferase isozymes involved in methylsulphide metabolism, was examined by a variety of methods, suggesting that their expression is regulated at both the transcriptional and post-transcriptional levels. Transcripts of all three genes, measured by quantitative reverse transcription PCR, were shown to be most abundant during growth on methanol with dimethylsulphide (DMS). Transcript levels were also high in media with CO or methylamines, but much lower with methanol. In contrast, translational fusions to mtsD showed high expression levels on CO or methanol with DMS, while the mtsF translational fusion showed highest reporter gene activity on methylamines with much lower expression on CO or methanol with DMS. The activity of mtsD and mtsF fusions was very low when the strains were grown in methanol or acetate. Expression of the mtsH fusion was not detected on any substrate, despite the presence of an mRNA transcript. The transcription start sites of all three genes were determined by 5'-RACE revealing large leader sequences for each transcript. Characterization of deletion mutants lacking putative regulatory genes suggests that MA0862 (msrF), MA4383 (msrC) and MA4560 (msrG) act as transcriptional activators of mtsD, mtsF and mtsH respectively.