Research Project
Genetic screens yielded many arg-6 mutants lacking either
AGK, AGPR, or both activities. These mutants were divided into two
complementation groups, and a third non-complementing group. Complementation
group A lacks AGK activity while complementation group B lacks AGPR activity.
Members of each group complement mutants of the opposite group but not
mutants within its own group. Mutants of the non-complementing group
lack both AGK and AGPR activities and can not complement group A or B mutants.
Further studies of these mutants revealed that some lack
acetylglutamate synthase (AGS) activity. AGS is the first enzyme
in the arginine biosynthetic pathway. This enzyme is encoded by the
arg-14 locus. This loss of activity appears to be a result of AGK-AGS
interaction, which involves an AGK C-terminal extension found in AGKs from
N. crassa and other fungi but not in AGKs from prokaryotes.
Furthermore,
the AGS-AGK interaction is assumed to be the cause of the concerted feedback
inhibition by arginine of both enzymes.
My project focuses on studying the reasons behind the
presence of the arg-6 polyprotein and its importance in the
survival of fungi.
The complex arg-6 locus in Neurospora crassa
encodes a polyprotein precursor for two mitochondrial early arginine
biosynthetic enzymes, acetylglutamate kinase (AGK) and
acetylglutamylphosphate reductase (AGPR). This polyprotein is
processed into two mature proteins as
it is translocated into the mitochondria. Processing involves cleavage
of the polyprotein at three different sites. Cleavage of the first
site, upstream of the proximal AGK, removes the mitochondrial targeting
sequence. The two other sites are upstream of the distal AGPR, and
cleavage removes a 20 amino acid region connecting both enzymes.