Post-transcriptional control of the differential expression of phosphoglycerate kinase genes in Trypanosoma brucei.

The genes for the cytosolic and glycosomal phosphoglycerate kinases (PGK) of Trypanosoma brucei are found in a compact tandem array together with a third PGK-related gene, expressed at low level. Expression of the two PGK genes is differentially regulated in the life cycle of T. brucei: the glycosomal PGK and its mRNA are abundant in the mammalian stage of the cycle but not in the insect stage, whereas the reverse is found for the cytosolic PGK and its mRNA. Nevertheless, our experiments indicate that the mRNAs for both isoenzymes are derived from a common precursor. Nuclease protection experiments using fragments cloned into single-stranded DNA vectors show the presence of low abundance RNA species running through one gene into the next. Indeed minor RNA species larger than the mature mRNAs are visible in overexposed RNA blots. Analysis of nascent RNA in a nuclear run-on assay indicates that the entire PGK gene array is transcribed at an equal rate throughout in both life cycle stages. We conclude that the PGK genes are part of one large multicistronic transcription unit, which is processed to yield the individual mRNAs with concomitant addition of the 5' 35-nucleotide mini-exon sequence, characteristic of all trypanosome mRNAs. It follows that the steady-state levels of the PGK mRNAs are controlled post-transcriptionally.