The in vitro transfer of DNA, was mainly developed by using bacteria as host cells (Bernard and Helinski 1980). Accordingly, suitable vector systems for recombinant DNA technology are predominantly of prokaryotic origin. Since gene cloning is becoming a more essential component of both fundamental research (e.g. gene expression, regulation) (Losson and Lacroute 1981) and applied research (e.g. production of therapeutants and other products relevant for biotechnology) (Johnson and Burnett 1978), the inclusion of eukaryotes into the spectrum of host organisms becomes necessary. One of the main reasons to explore eukaryotes for such research is that bacteria, when used as hosts for cloning of eukaryotic DNA, may cause difficulties, such as failure in replication or expression of the foreign DNA; complications when extracting the product; or instability of the transformants (MacLeod 1980).
In order to avoid complications of this kind, it is desirable for the cloning of eukaryotic DNA to be restricted to eukaryotic systems only, i.e. eukaryotes providing both the host and the vector. This concept requires:
- a vector which guarantees replication to a high copy number and expression of the foreign DNA, 2. a selective system which allows identification of the vector in the host cell, 3. compatibility and stability of the vector and the DNA which is to be cloned in the host.
* In this paper the term mitochondrial (mt) DNA is inclusive for all hereditary determinants present in the mitochondrion, such as the circular mitochondrial chromosome and mitochondrial plasmids, whether integrated in the mitochondrial chromosome or not. Autonomously replicating sequences are referred to as replicons