Molecular aspects of the cell cycle and encystment of Acanthamoeba.

Evidence for subdivision of the cell cycle of Acanthamoeba into ultradian biochemical cycles is accumulating, and a linkage between these cycles and the length of the cell cycle is possible. The DNA replication cycle differs with the method of assay: no G1 phase is found in asynchronous cultures, and a long G1 phase is found in synchronous cultures. Encystment most likely occurs from G2, but whether it is limited to a portion of this phase is not clear. Encystment-enhancing factors are released by Acanthamoeba castellanii and Acanthamoeba palestinensis, and encystment can be induced by monoclonal antibodies to plasma membrane proteins. Likewise, encystment can be induced by inhibitors of polyamine synthesis, especially diamidines that inhibit S-adenosylmethionine decarboxylase, but inhibition of this enzyme is not necessarily responsible for differentiation. Studies on the regulation of gene expression during encystment have focused on actin and the ribosomal RNA transcriptional unit.