ABRUPT RNA CHANGES PRECEDE THE 1ST CELL-DIVISION DURING THE DIFFERENTIATION OF TRYPANOSOMA-BRUCEI BLOOD-STREAM FORMS INTO PROCYCLIC FORMS IN-VITRO

We have monitored the timing of DNA and RNA synthesis during the synch ronous differentiation of Trypanosoma brucei bloodstream forms into pr ocyclic forms in vitro. Both are triggered after a lag period of 4 h a nd reach a first peak after 9 h. The division of the kinetoplast prece des that of the nucleus by about 4 h. The first cell divisions are obs erved 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 progr essively decreases as cell division progresses. The increase in RNA co ntent cannot be due solely to accumulation of rRNA since it is also ob served for mRNAs such as actin. The VSG mRNA has almost disappeared wi thin 2 h, while the procyclin mRNA accumulates soon after the triggeri ng of differentiation, with a strong peak between 4 and 6 h. At this m oment, the amount of procyclin mRNA per cell is at least 20-fold highe r than in established procyclic culture forms. The loss of the VSG and the appearance of procyclic-specific proteins essentially occur befor e the first cell division. These observations contrast with the progre ssive transition observed when monomorphic slender forms are induced t o transform under the same conditions.