After the first discovery that it was possible to carry out genetic analysis in E. coli, genetic exchange was soon demonstrated in other phylogenetically distant bacteria, and harnessed for experimental purposes. Against this background, several scientists began in the mid-1950s to look for evidence of genetic exchange in Streptomyces (reviewed by Hopwood 2007). There were three main reasons why this particular group of bacteria attracted attention. Firstly, it had become obvious that streptomycetes were a rich source of antibiotics, so there was a real possibility that success in developing genetic systems would find long-term utility. Secondly, these abundant, widespread, ecologically significant soil organisms were phylogenetically distant from E. coli and other genetically studied bacteria, and had an unusual mycelial growth habit like that of fungi (Fig. 1) (in fact, some people still incorrectly thought of them as intermediate between bacteria and fungi, or even as fungi). It was therefore felt that they should, on the one hand, exhibit novel features and, on the other, help to reveal what was universally true of bacteria. Thirdly, they were easy to grow at a reasonable rate on defined media, and went through a stage in their life cycle in which, as spores, they had a single copy of the genome, allowing the progeny of crosses to be classified readily; they were thus potentially experimentally convenient.
The greatest success was had with Streptomyces coelicolor A3(2), the organism chosen by D.A. Hopwood (Hopwood 1999). His earliest papers showed that random segments of its chromosome could be transferred between strains by conjugation, with both participants being able to act as donor or recipient (the crosses were "non-polar") and without obvious preference for particular chromosome segments (Hopwood 1967). Subsequent work revealed the involvement of a complex fertility system, involving plasmids, which is described in the next section. It was from this work that the unusual nature of some Streptomyces plasmids began to emerge, culminating in the 1980s with the revelation that many of them are linear.
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