Modulation of CTP: phosphocholine cytidylyltransferase activity in choline-sensitive and choline-resistant tobacco cells
1993
Gawer, M. | Chervin, D. | Guern, N. | Mazliak, P.
Microsomes of cultured tobacco cells exhibited a CTP:phosphocholine cytidylyltransferase (CT) activity which was involved in the regulation of phosphatidylcholine (PC) biosynthesis. Some properties of this cellular activity measured in vitro are reported, especially its changes over a 35-day culture period. The changes were modulated by the presence of choline in the culture medium and differed between choline-sensitive (ChS) and choline-resistant (ChR) cell lines. In ChS cells a high CT stimulation, which was partially cycloheximide insensitive, occurred during the lag phase and the first wave of cell divisions, and was followed by an abrupt drop during the final cell divisions. The total activity did not parallel the microsomal protein content of the cells. The early stimulation of CT activity was also found in ChS cells which were cultivated in the presence of choline but was not as high as in cells cultivated in the absence of choline. CT activity, as well as microsomal protein content of the cells, remained high in the stationary growth phase of ChS cells cultivated on a choline-supplemented medium. In marked contrast, in ChR cells, routinely cultivated on choline, CT activity was constant so that the total activity only varied in accordance with microsomal protein content during the exponential and stationary growth phases. A non-specific nucleotidase catalysing the hydrolysis of CDP-choline exhibited an activity opposite to the overall activity of CT. In conclusion, PC biosynthesis appears to be controlled either by de novo synthesis of CT either by a reversible association of this protein to membranes, or by variable levels of nucleotidase and PC precursors. Alternative pathways concerned with ethanolamine methylation should also be considered.
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