A comparison of the proteins of the amoebal and plasmodial phases of the slime mould, Physarum polycephalum.

1. The proteins of amoebae and plasmodia of strain CL of Physarum polycephalum have been compared by two-dimensional gel electrophoresis. Both forms of the organism were labelled by growth on formalin-killed bacteria labelled with [35S]sulphate, [3H]lysine or [14C]lysine. Plasmodia were also labelled from radioactive lysine in the medium. 2. Of 306 relatively abundant proteins examined, 26% were phase-specific, that is they were found only in amoebae or in plasmodia. About a quarter of these apparent differences in gene expression may be due to minor changes in charge and/or size. 3. Amongst the 74% of the proteins present in both amoebae and plasmodia, there are substantial differences in differential rates of synthesis and these have been measured for a representative set of proteins by a double-label procedure.