COORDINATE REPRESSION OF THE SYNTHESIS OF FOUR HISTIDINE BIOSYNTHETIC ENZYMES BY HISTIDINE

Abstract

Enzyme repression is the term used to describe the regulation of the rate of synthesis of the enzymes of a biosynthetic pathway by the size of the pool of the end product of that pathway. Data are presented on investigations concerned with the histidine biosynthetic system from Salmonella; histidine synthesis involves 6 enzymes, 4 of which were studied here: (1) IGP dehydrase, (2) IAP transaminase, (3) histidinol phosphate phosphatase, and (4) histidinol dehydrogenase. Histidine, acting as a repressor, seems to affect the synthesis of these enzymes to the same extent. The authors propose that this phenomenon be called "coordinate regression." This repression appears to be independent of the amount of substrate of each enzyme present in the cells since mutants from all genetic classes show these coordinate changes in enzyme levels. The repressor may block enzyme synthesis at the gene level. Histidine synthesis is also regulated at a substrate level in Salmonella when excess histidine is present; this feedback control results from the inhibitory effect of the end product of a pathway on the activity of an early enzyme of the pathway.