The experimental work and theoretical discussion presented
here suggests the following main conclusions.1. Argininosuccinase may be detected in extracts of a standard
wild -type (SLA) and measurements may be made of some of the kinetics
of its catalysis. I:t is considered likely, from these kinetic data
and from the information on the behaviour of the enzyme on hydroxylapatite
gel and on electrophoresis, that the enzyme is a single protein species.2. A report is given of an examination of some of the arg-10 mutants
and of a heterokaryon between two of them.3.. The production and genetics of revertants of some of these arg-10
mutants, is described and it is suggested that at least two of the revertants
(362r-1 and 362r-2) are the result of mutation(s) at or close to the
arg-10 locus.4. The argininosuccinase formed by 362r-1 is described and it is
proposed that the differences between this enzyme and that of the
original wild-type (SLA) may be explained as the result of an alteration
to the structure of the enzyme involving the active site.5. The argininosuccinase formed by 362r-2 is described and it is
proposed that this enzyme differs both from that found in SLA and in
362r-1 and that this is also a reflection of an alteration in the
structure of the active site of the enzyme.6. Some of the properties of argininosuccinase from K32 3-revertants are described and it is suggested that they may not differ in any
way from the enzyme of SLA.7. A discussion is given of the growth of Neurospora crassa in
culture and it is concluded that the growth -curves of the organism
are complex and depend on the culture conditions.8. Measurements of argininosuccinase, argininosuccinic acid and
arginine in cultures of SLA and 362r-1 are reported and the results
obtained are explained in terms of a kinetic model of the arginine
pathway in vivo. It is suggested that the concentration of arginine
must always be independent of argininosuccinase concentration (at
steady-state).9. In a discussion of the experiments and theory presented, the
thesis is proposed that there are at least two major classes of
catalyses, 'buffered" and "unbuffered", and that the genes affecting
the enzymes concerned with these two types of catalyses will have
distinct properties
