Abrupt RNA changes precede the first cell division during the differentiation of Trypanosoma brucei bloodstream forms into procyclic forms in vitro.

We have monitored the timing of DNA and RNA synthesis during the synchronous differentiation of Trypanosoma brucei bloodstream forms into procyclic forms in vitro. Both are triggered after a lag period of 4 h and reach a first peak after 9 h. The division of the kinetoplast precedes that of the nucleus by about 4 h. The first cell divisions are observed after 10 h, and the cell number is doubled after 20 h. The total RNA content per cell increases sharply between 4 and 10 h, then progressively decreases as cell division progresses. The increase in RNA content cannot be due solely to accumulation of rRNA since it is also observed for mRNAs such as actin. The VSG mRNA has almost disappeared within 2 h, while the procyclin mRNA accumulates soon after the triggering of differentiation, with a strong peak between 4 and 6 h. At this moment, the amount of procyclin mRNA per cell is at least 20-fold higher than in established procyclic culture forms. The loss of the VSG and the appearance of procyclic-specific proteins essentially occur before the first cell division. These observations contrast with the progressive transition observed when monomorphic slender forms are induced to transform under the same conditions.