Regulation of synthesis of serine hydroxymethyltransferase in chemostat cultures of Escherichia coli.

Serine hydroxymethyltransferase was synthesized as a constant fraction of total protein of Escherichia coli over a wide range of specific growth rates. This was observed in all strains when grown in glucose-limited chemostat cultures; in thymine-requiring mutants during thymidine-limited growth; and in met A and met B auxotrophs, defective in homocysteine biosynthesis, during methionine-limited growth. This behavior has been referred to by others as "metabolic control." In addition, the synthesis of serine hydroxy-methyltransferase was subject to specific active control mechanisms, which responded to the needs of the cell for purine biosynthesis, methylation reactions, as well as to serine limitation. Under purine limitation, the rate of enzyme synthesis increased with decreasing growth rate, that is with increasing purine limitation. During methionine-limited growth of met E and met F auxotrophs (mutants unable to methylate homocysteine) the rate of enzyme synthesis increased with a decrease in specific growth rate from 0.65 to 0.30 h-1 but declined with further decrease in growth rate. Under serine limitation the rate of enzyme synthesis remained proportional to the growth rate, but at a rate twice that observed in unrestricted or glucose-limited growth. When purines were added to unrestricted or glucose-limited cultures, the rate of enzyme synthesis decreased by 40%, but remained proportional to growth rate. Addition of methionine or serine alone had no effect.