Methylation is an early and necessary step in the sporulation programme of the slime mold Physarum polycephalum.

Formation of sporangia can be induced in starved macroplasmodia of Physarum polycephalum by illumination. This process of morphogenesis which starts 9 h after the needed period of illumination can be prevented by the competitive inhibitors of methyltransferases S-adenosyl-homocysteine or L-ethionine, applied either directly by microinjection into the plasmodia or by addition to the medium. Because 5-azacytidine (aza-C) or 5-aza-2'-deoxycytidine (aza-dC) also prevent sporulation (in contrast to cytidine, 8-azaguanine (aza-G) or 6-aza-uridine (aza-U) it is suggested that DNA is the substrate for methylation. The injection technique allows one to determine the period of methylation (i.e. between the 3rd and 4th h of the cell differentiation process after the induction by illumination). Based on the correlation between methylation and genome expression, it is suggested that some genes must be repressed by methylation during this period.