ACTIN MESSENGER-RNA LEVELS AND ACTIN SYNTHESIS DURING THE ENCYSTATIONOF ENTAMOEBA-INVADENS

Parasitic amebas propagate among hosts through cysts, the resistant fo rms in their life cycle. In spite of their key role in infection, litt le is known about the encystation process and the mechanisms involved in reaching this stage. Two features drastically affected by encystati on are motility and cell shape, both of which are determined by the cy toskeleton, composed mainly of actin in these organisms. Therefore, we studied the occurrence and relative levels of actin and actin synthes is during encystation of Entamoeba invadens. Using a cDNA actin probe obtained from a library off. histolytica and a monoclonal antibody aga inst actin, we found that, while the total actin levels sharply decrea se as encystation proceeds, the levels of actin mRNA are reduced only in mature cysts. Moreover, actin synthesis does not take place in prec ysts and the later stages of cyst formation. In contrast, the levels o f other proteins remain stable in trophozoites, precysts and cysts, an d stage specific peptides are actively synthesized in precysts. The re sults indicate that encystation is accompanied by a preferential down- regulation of actin synthesis and a decrease in actin levels. The reor ganization of the cytoskeleton occurring as trophozoites transform int o round, quiescent cells, could be a regulatory factor in the observed changes.