POLYSOMAL, PROCYCLIN MESSENGER-RNAS ACCUMULATE IN BLOOD-STREAM FORMS OF MONOMORPHIC AND PLEOMORPHIC TRYPANOSOMES TREATED WITH PROTEIN-SYNTHESIS INHIBITORS

The major surface antigen of insect stage (procyclic and epimastigote form) Trypanosoma brucei is termed procyclin or procyclic acidic repet itive protein (PARP). Procyclin/PARP is not expressed in bloodstream f orm parasites, which are coated instead with the variant surface glyco protein (VSG). Although procyclin/PARP protein is not present and the mRNA is barely detectable, procyclin/PARP genes are transcribed at thi s life cycle stage. We examined the mechanism for down-regulation of p rocyclin/PARP expression in bloodstream trypanosomes by using protein synthesis inhibitors to effect accumulation of procyclin/PARP transcri pts. We show that the accumulation is not due to increased transcripti on of procyclin/PARP genes. Further, transcripts which accumulate unde r these conditions are of mature size, polyadenylated and polysome-ass ociated indicating that normally, in bloodstream trypanosomes, down-re gulation of procyclin/PARP expression is exerted either during transcr ipt processing or at the level of mRNA stability. A comparison of the inhibitor-induced accumulation of procyclin/PARP transcripts in bloods tream forms of monomorphic and pleomorphic cell lines of trypanosome s tock EATRO 795 shows that accumulation occurs with similar kinetics in both cell lines.