The synthesis of serine and Leuconostoc citrovorum factor by cell suspensions of Streptococcus faecalis R

Abstract

Cells of S. faecalis R, harvested from a medium deficient in folic acid and vitamin B6, synthesize serine when incubated in a buffered mixture of glycine, formate, glucose, pyridoxal and pteroyl-glutamic acid. Omission of any one of these substances reduces serine formation to one-tenth or less. Serine was estimated by specific microbiological assay, and further identified in typical experiments by chromatography on Dowex resin. Formate is not replaced by other substances active as one-carbon donors with animal tissues. N5 - Formyl - 5:6:7:8 - tetrahydropteroyl-glutamic acid (leucovorin) and N10.formylpteroylglutamic acid replace pteroylglutamic acid, though the former was somewhat less active. Cells first treated in buffered glucose with leuco-vorin or with pteroylglutamic acid plus formate (and then washed) have greater synthetic ability than cells similarly treated with pteroylglutamic acid alone; such synthesis is no longer dependent on added folic acid. Leuconostoc citrovorum factor is formed concurrently with serine in the complete system. Glycine and pyridoxal are not required for this synthesis. Synthesis of both serine and L. citrovorum factor is inhibited (competitively with pteroylglutamic acid) by Nl[degree]-methylpteroic acid. Serine formation by cells first treated with leucovorin or pteroylglutamic acid plus formate is insensitive to the analogue as is that of untreated cells when leucovorin replaces pteroylglutamic acid in the test system. It is concluded that pteroylglutamic acid is transformed to a form of folic acid similar to but probably not identical with leucovorin before it is active in serine synthesis by this organism. Serine is required for growth of S. faecalis R on a medium not containing folic acid or vitamin Bg; it can be replaced by glycine (at high concentration) only if sources of these vitamins are added, but formate is not required.