Biosynthesis by Escherichia coli of active altered proteins containing selenium instead of sulfur

Abstract

1. 1. Selenomethionine can completely replace methionine for the normal exponential growth of a methionine-requiring mutant of E. coli. β-Galactosidase is formed under these conditions. 2. 2. Selenium cannot entirely replace sulfur for the growth of E. coli. However, in the presence of the glutathione sulfur reservoir or with traces of sulfate, selenium partially replaces sulfur. Hydrolyzates of the residual protein fraction having incorporated radioselenium contain a radioactive material with chromatographic properties similar to those of cysteine. The incorporation of selenium is proportional to the increase in bacterial mass. No selenoglutathione is formed. Selenium is also incorporated into the alcohol-soluble proteins, but cannot be transferred from it to the residual proteins during sulfur starvation. Active β-galactosidase cannot be formed at the expense of the alcohol-soluble proteins whether selenium is present or absent. 3. 3. The results reported here and elsewhere demonstrate that the amino acid composition of the proteins may be influenced by the presence of exogenous amino acids or their analogs, and suggest that some variation in the amino acid composition of a given protein may be the rule rather than the exception.