Transport of biosynthetic intermediates: regulation of homoserine and threonine uptake in Escherichia coli

Abstract

Homoserine is transported by a single system that it shares with alanine, isoleucine, leucine, phenylalanine, threonine, valine and perhaps cysteine, methionine serine and tyrosine. The regulation of this transport system was investigated and it was found that alanine, isoleucine, leucine, methionine, and valine each repress the homoserine transporting system. From the concentration resulting in 50% repression of this transport system and the maximal amount of repression, the amino acids were ranked according to their effectiveness in repressing homoserine transport (in decreasing order): leucine>methionine>alanine>valine>isoleucine. The exponential rate of decrease in transport capacity after leucine addition equals the exponential growth rate of the culture, and protein synthesis is necessary for the derepression seen when leucine is removed. threonine, in addition to using the above system, is transported by a second system shared with serine. The authors present further evidence for this serine threonine transport system and show that it is not regulated like the homoserine transport system.