Organization of the histidine-3 region of Neurospora.

Abstract

The histidine-3 region of Neurospora specifies the structure of three enzymes of the histidine biosynthetic pathway viz., PR-AMP 1,6-cyclohydrolase, PR-ATP pyrophosphohydrolase, and histidinol dehydrogenase. Point mutations in this region may either affect individual enzyme activities or all of the three activities simultaneously. Attempts have been made to fractionate the three enzymes in order to understand the organization of the his-3 region. Gel-filtration on Sephadex G-200 revealed a single peak of activity for pyrophosphohydrolase and dehydrogenase. The two enzymes also emerged as one peak on chromatography through a DEAE-cellulose column eluted with linear salt gradient. Sedimentation characteristics of cyclohydrolase, pyrophosphohydrolase, and dehydrogenase were identical on sucrose density-gradient centrifugation. The molecular weight of this aggregate was estimated to be about 140,000. Other enzymes of histidine biosynthesis could be readily separated by these methods. Furthermore, mutational events occurring at several points across the genetic map seem to influence the three activities simultaneously. These results suggest that the polypeptides coded by the his-3 region are organized into a functional aggregate or multi-enzyme complex. Analysis of existing four-point cross data indicates that recombination within his-3 region is polarized. This polarity, according to the model proposed by Bernstein (1964), is towards the centromere and proceeds in a direction opposite to translation as deduced from complementation.