Sites of protein synthesis in Nucleoli of root meristematic cells of Allium cepa as shown by radioautography with [3H]arginine.

The interphase nucleolus in root meristematic cells of Allium cepa may be divided into four regions, three of which are always present: the fibrillar, granular and lacunar regions, while the fourth or vacuolar region may be missing. The sites of protein synthesis in nucleoli were investigated by means of light and electron-microscope radioautography after a 5-min immersion of the roots in a solution of [H]arginine. The radioautographs of interphase nucleoli showed many silver grains over both the fibrillar and the granular regions. Occasional silver grains were also recorded over, or close to, the lacunar regions, but none were over the vacuolar regions. A 15-min chase period did not change the radioautographic pattern. It is concluded that the three permanent regions of the interphase nucleoli, namely the fibrillar, the granular and the DNA-containing lacunar regions, are sites of protein synthesis.