DIATOM MINERALIZATION OF SILICIC ACID. II. REGULATION OF Si (OH) 4 TRANSPORT RATES DURING THE CELL CYCLE OF NAVICULA PELLICULOSA 1

Synchronized populations of Navicula pelliculosa (Bréb.) Hilse show a 10‐fold increase in Si(OH) 4 transport rate during traverse through the cell division cycle. The transport activity pattern is similar to a “peak enzyme.” Kinetic analysis showed there was a significant change in K s values, indicating increased “affinity” for Si(OH) 4 as cells neared maximal uptake rates. However, the dramatic changes in transport rate at various cell cycle stages were also reflected by alterations in the V max , values of the transport process, suggesting a change in the number of functional transport “sites” in the plasma membrane. Cells in the wall forming stage, arrested from further development by Si(OH) 4 deprivation, maintained high transport rates for as long as 7 h. The rates decreased rapidly if protein synthesis were blocked or if Si(OH) 4 was added, the latter allowing the cells to traverse the rest of the cycle. The half‐life of the transport activity ranged from 1.0 to 2.2 h when protein synthesis was inhibited at various cell cycle stages and during the natural decline of activity late in the cycle. The transport system appears to be metabolically unstable as is typical for a “peak protein.” The rise in transport rate through the cell cycle did not depend on the presence of Si(OH) 4 in the medium; therefore, the transport system does not appear to be induced by its substrate. The rise in transport is also observed in L:D synchronized cells developing in the presence of Si(OH) 4 ; neither does the transport system appear to be derepressed. The transport rate was strongly cell cycle‐stage dependent; the data appeared to fit the “dependent pathway” model proposed by Hart‐well to explain oscillations in enzyme synthesis during the cell cycle.