The Model Organisms Used In This Study.

Two model organisms which assimilate nitrate were used in this research work. First, the haploid ascomycetous filamentous fungus

Aspergillus nidulans and second, the diploid higher plant Arabidopsis

thaliana.

1.1.1 Aspergillus nidulans.

The eukaryotic fungus Aspergillus nidulans grows on relatively cheap laboratory media requiring only basic growth requirements i.e., an organic carbon and inorganic nitrogen source, water, oxygen, and trace elements. The organism is considered to be relatively easy to handle and safe to deal with, in terms of laboratory research work (Pontecorvo et. al., 1953; and for a recent review see Martenilli, 1994 and refernces therein). Another attraction is that A. nidulans

possesses a relatively small genome compared to most other eukaryotes, and is distributed over only eight separate chromosomes (Brody and Carbon, 1989) or linkage groups (Clutterbuck, 1994). Moreover, A. nidulans possesses a spectrum of genetic systems which can be m anipulated including nuclear, organellar (mitochondrial DNA) and extra organellar (plasmids) which provides an opportunity for the study, characterisation and interactions between nuclear.

organellar and the extra organellar genetic systems. Also A. nidulans

has a variety of cell forms (unicellular, multicellular as well as vegetative asexual and sexual) providing an unique opportunity for the study of eukaryotic developmental and differentiation problems using combined genetical and biochemical approaches. The occurrence of haploid, heterokaryon and diploid cells in a relatively short life cycle, during sexual, asexual and parasexual cycles is very useful (Timberlake and Clutterbuck, 1994). For instance the sexual stage can be used normally for conventional genetic mapping studies, deletion analysis and the examination of inheritance of mutations. The asexual uninucleated spores are perfect for mutagenesis and the fact that the easily seen pigment mutations can be used as markers in genetic crosses. Another most important feature is the occurrence of balanced heterokaryons, in which nuclear migration occurs freely. Finally, the parasexual stage is mainly used for complementation studies, dominance tests, centromere mapping, and the assignment of newly isolated mutations to linkage groups. Additionally, most fungi have the ability to form balanced heterokaryons, through which nuclear migration occurs freely. Heterokaryons can be used for parasexual crosses, which facilitates the solving of many problems such as allelelism questions by complementation tests. Also, this experimental approach helps in the useful analysis of regulatory mutants of gene expression. Fungi in general, are similar to higher eukaryotic organisms in their chromosome structure (nucleosomal organisation, and the presence of histone proteins), mRNA processing.

transcriptional machinary, and therefore are likely to be similar in gene expression mechanisms. However, fungi differ from higher eukaryotes in the lack of abundance of repeated DNA sequences. Moreover, the simplicity of isolating large number of mutants, the development of an efficient transformation protocols and the construction of contiguous physical maps, all help to gather more and more important information. Additionally, the isolation of an array of genes of both scientifical and commercial interest, generates great enthusiasm for the isolation of most genes that lie in this relatively small genome in the near future.

In summary, the main useful features of A.nidulans, is that it is a lower eukaryotic organism which is a useful and convenient model system for the study of biological systems, (for a treatise see Martinelli and Kinghorn, 1994 and the various reviews therein, also previously reviewed by Arst, 1981, 1983, 1984, Johnstone, 1985; Mishra, 1985; Hynes, 1986).

1.1.2 Arabidopsis thaliana.

The diploid (2n=10, i.e the relatively small genome is distributed over only five pairs of separate chromosomes) higher plant species Arabidopsis thaliana belongs to the genus Arabidopsis

which in turn belongs to the mustard or crucifer family (cruciferae).

Arabidopsis thaliana is widely used for studies of classical and

molecular genetics. The utility of A thaliana for experiments in molecular biology comes in part from its many advantages including

its relatively small size, short life cycle, screening of large number of mutants, its contribution to the ease of use of this plant in the laboratories and its small nuclear genome. All of these features have reduced the expense and effort required for many types of experiments in molecular genetics. Such correlates as its relative lack of repetitive DNA sequences, reduced copy number of multigene families and reduced size of introns and intergenic spacers (some 60% of the genome consists of single-copy DNA sequences, most of which are genes or associated sequences). These properties facilitate a series of different types of experiments in molecular genetics and allow the cloning of many Arabidopsis genes by methods that would be difficult or even impossible if the genome were larger or more typical in its content of repetitive sequences. The structure of individual genes, the structure of chromosomes, the genetic properties, and the overall complement of genes in the genome are typical of those of other flowering plants. Since the above mentioned characters Arabidopsis thaliana has been regarded as a useful and therefore popular model organism for investigating a wide range of research topics in plant molecular biology. Arabidopsis differs from several of the other model organisms in at least one key respect.

Arabidopsis is closely related to the species it models. In this respect

all angiosperms species are thought to have evolved from a common ancestor, in which they share similar life styles, environmental challenges, and modes of reproduction. An Arabidopsis gene may be expected to functionally replace a homologue in many other flowering plants and may be used as a cross-species hybridisation probe to detect

and isolate the corresponding gene. One of the implications of this high degree of similarity between the model and the modelled is that all aspects of Arabidopsis biology-development, metabolism, biochemical, environmental, and so are worthy of investigation because of the broad applicability of the gathered information (reviewed in Meyerowits and Somerville, 1994).

1.2 The Nitrate Assimilation Pathway.