SUMMARY: when growing under the conventional synchronizing conditions of batch culture and intermittent illumination shows periodic changes in the rate of SO uptake and in the specific rate of S incorporation into protein relative to total protein. There are also periodic changes in the rates of synthesis of individual proteins including several chloroplast membrane proteins and of tubulin, which is synthesized maximally prior to the initiation of division events and is not synthesized during cytokinesis. Activities of citrate synthase and aspartate transcarbamoylase enzymes increase discontinuously under synchronizing conditions. These periodic events are not part of a developmental programme which is essential for division, since the periodicity disappears under constant environmental conditions. Synchronously dividing cells then show smoothly increasing rates of SO uptake into cells and of incorporation into protein. There is a constant specific rate of isotope incorporation, relative to total protein, which correlates with the exponential accumulation of protein through the cell cycle. The 300 most abundantly labelled proteins are synthesized continuously through the cell cycle and this accords with the continuous accumulation of citrate synthase and aspartate transcarbamoylase enzyme activities. Tubulin synthesis is continuous under constant growth conditions and therefore periodic increases in tubulin concentration are not an essential part of mitotic spindle or phycoplast formation. Of the sedimentable proteins only one of the 200 most abundantly labelled shows a cessation of synthesis. A previously reported periodic synthesis of chloroplast membrane proteins is not essential for chloroplast assembly in dividing cells. The significance of these data is discussed in relation to cell cycle control and it is proposed that three categories of periodic event should be recognized. Periodic syntheses of proteins that are directly involved in division processes may be classed as primary cell cycle events if they accompany the cell cycle under all growth conditions, and these events may include division-initiating processes. If periodic synthesis of such proteins occurs only under some growth conditions then their synthesis is classed as a secondary cell cycle event. Discontinuous synthesis of proteins that do not contribute directly to division processes and are only synthesized discontinuously during or in recovery from changing environmental conditions, is considered a tertiary cell cycle event, but by adapting cell structure and metabolism to environmental conditions such events may indirectly sustain division processes. It is suggested that the periodic synthesis of tubulin under synchronizing conditions is consistent with the operation of a secondary cell cycle control, but that the periodic increases in the activity of a respiratory and biosynthetic enzyme and the periodic synthesis of numerous individual unidentified proteins, under changing conditions in synchronous culture are only tertiarily related to the cell cycle.


Article metrics loading...

Loading full text...

Full text loading...


Most cited this month Most Cited RSS feed

This is a required field
Please enter a valid email address
Approval was a Success
Invalid data
An Error Occurred
Approval was partially successful, following selected items could not be processed due to error