Injection of poly(β-l-malate) into the plasmodium ofPhysarum polycephalumshortens the cell cycle and increases the growth rate.

Poly(β-l-malate) (PMLA) has been reported as an unconventional, physiologically important biopolymer in plasmodia of myxomycetes, and has been proposed to function in the storage and transport of nuclear proteins by mimicking the phospho(deoxy)ribose backbone of nucleic acids. It is distributed in the cytoplasm and especially in the nuclei of these giant, multinucleate cells. We report here for the first time an increase in growth rate and a shortening of the cell cycle after the injection of purified PMLA. By comparing two strains ofPhysarum polycephalumthat differed in their production levels of PMLA, it was found that growth activation and cell cycle shortening correlated with the relative increases of PMLA levels in the cytoplasm or the nuclei. Growth rates of a low PMLA producer strain (LU897 × LU898) were increased by 40–50% while those of a high producer strain (M3CVIII) were increased by only 0–17% in comparison with controls. In both strains, shortening of the cell cycle occurred to a similar extent (7.2–9.5%), and this was associated with similar increases in nuclear PMLA levels. The effects showed saturation dependences with regard to the amount of injected PMLA. A steep rise of intracellular PMLA shortly after injection was followed by the appearance of histone H1 in the cytoplasm. The increase in growth rate, the shortening of the cell cycle duration and the appearance of H1 in the cytoplasm suggest that PMLA competes with nucleic acids in binding to proteins that control translation and/or transcription. Thus, PMLA could play an important role in the coordination of molecular pathways that are responsible for the synchronous functioning of the multinucleate plasmodium. [ABSTRACT FROM AUTHOR]