Studies on the biochemistry and fine structure of silica shell formation in diatoms

In diatoms, the siliceous cell walls are enveloped by an organic component which includes 4-hydroxyproline and 3,4-dihydroxy-L-proline. The formation of these two amino acids were studied in Nitzschia angularis in Si-starvation synchrony. Both appear to arise from peptidyl proline. Its conversion to peptidyl hydroxyproline was shown in cell-free extracts and in kinetic studies using [¹⁴C]proline. Two lines of evidence indicate that dihydroxyproline does not arise from the further hydroxylation of peptidyl hydroxyproline: First, there was a lag of several minutes between the incorporation of [¹⁴C]proline into protein and the appearance therein of [¹⁴C]hydroxyproline but no such lag for the appearance of dihydroxyproline. Second, ,-dipyridyl blocked the formation of hydroxyproline, but not of dihydroxypyroline, from peptidyl proline. Cell walls made in the presence of dipyridyl differed little in overall chemical composition from walls made in its absence and were morphologically identical. [¹⁴C]dehydroproline was rapidly metabolized in the cells, with [¹⁴C]dihydroxyproline a prominent product. Studies of the conversion of [¹⁴C]proline to [¹⁴C]hydroxyproline at various stages of wall formation showed an increased synthesis of [¹⁴C]dihydroxyproline at the end of cell separation.