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Molecular constituents of the replication apparatus in the plasmodium of Physarum polycephalum: identification by photoaffinity labelling

lnstitut fur Biophysik und physikalische Biochemie der Universitat, D-93040 Regensburg, Germany
Novosibirsk Institute of Bioorganic Chemistry Siberian Division of the Russian Academy of Sciences,630090 Novosibirsk, Russia
Institute of Cytology and Genetics3, Siberian Division of the Russian Academy of Sciences,630090 Novosibirsk, Russia

⁴2Author for correspondence: Eggehard Holler. Tel: +49 941 943 3030. Fax: +49 941 943 2813.e-rnail: Eggehard.Holler@ biologie.uni-regensburg.de

ABSTRACT

SUMMARY: The plasmodium of Physarum polycephalum has long been considered a model system for syncytically growing cells, but important details of the DNA replication apparatus, such as the DNA polymerase E and other replication factors, have not been detected. In this study, a new variation of photoaffinity labelling and immunoblotting was used to detect DNA polymerases and other factors in nuclear extracts of P. polycaphalum. Proteins were specifically cross-l inked with photoreactive arylazido-dCMP residues incorporated during extension of template-primer DNA. The DNA synthesized in situ was labelled. After nucleolytic removal of protruding DNA, the proteins were separated by SDS-gel electrophoresis, electroblotted on membranes and subjected to autoradiography. The ,,and β-like DNA polymerases were labelled, as were histones and replication-factor-like proteins. Cytoplasmic extracts were devoid of these species. Abundant proliferating-cell nuclear antigen and replication protein A large subunit were labelled and found to be of unusual mass. A number of subunits of purified DNA polymerase holoenzymes were labelled. In contrast, only the DNA-polymerizing subunits could be labelled in nuclear extracts. Higher-order complexes in the nuclear extract may make subunits inaccessible to photo-cross-linking. Complex formation is promoted by β-poly(~-malate), a plasmodium-specific putative storage and carrier molecule that supports DNA replication in the synchronized nuclei. Percoll, a polyvinylpyrrolidone-coated colloidal silica, partially disrupted these complexes. A 200 kDa fragment of DNA polymerase E and a 135 kDa β-like DNA polymerase did not participate in the complexes, suggesting functions unlike those of the other polymerases. DNA polymerase molecules were intact during proliferation of plasmodia, but were nicked before their clearance from the nuclei at growth arrest.

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