## 1 Introduction  
Horizontal gene transfer (HGT) is a widespread mechanism by which organisms from all three domains of life are able to exchange genetic material [1]. This trait allows them to occupy new ecological niches or to acquire novel virulence factors or antibiotic resistance [2, 3]. One of the three main methods for HGT is natural transformation, which has been observed to occur both in bacteria and archaea under conditions that can vary considerably for different transformable species [4, 5]. Natural genetic transformation is a parasexual process that requires an extracellular DNA molecule and a competent recipient cell. Since its discovery in 1928 [6], many bacterial species from several taxonomic groups have been shown to bear this trait. Thus far, most of the studies aiming to unveil the molecular mechanism behind natural transformation have been conducted on pathogens such as _Neisseria gonorrhoeae_[7], _Haemophilus influenzae_[8], _Vibrio cholerae_[9], and _Streptococcus pneumoniae_ or model organisms such as _Bacillussubtilis_[10] and _Thermus thermophilus_[11]. While progress has been made regarding the molecular details in Firmicutes (low-GC Gram-positive bacteria) [12; 13], almost nothing is known yet for the second major Gram-positive phylum and the Actinobacteria (high-GC Gram-positive bacteria), which are among important pathogens such as _Mycobacterium tuberculosis_. This is remarkable, considering that _Microococcus luteus_ (_M. \"lysodetikkus\"_) was among the first bacteria used for research on this topic [14; 15]. Most bacterial species that use natural transformation rely on similar DNA uptake machineries [5; 16]. Certain core genes involved in this process have been shown to be conserved across phylogenetically distant taxa. All transformable bacteria studied thus far harbor the transmembrane channel ComEC and an extra-cytoplasmic DNA-binding protein such as ComEA in _B. subtilis_[4; 17], which serves as a receptor to deliver dsDNA to the highly conserved ComEC [18]. The competence (_com_) genes _comEA_ and _comEC_ on the _M. luteus_ genome overlap and are co-transcribed genes, and both have been shown to be essential for transformation [16]. The expression of _comEC_, _comEA_, and other competence genes is often a tightly regulated process that is triggered in certain physiological states or under specific stress conditions. Competence gene expression is mainly activated during the early exponential phase in _S. pneumoniae_ and strictly during the stationary phase in _B. subtilis_[19; 20]. Although the molecular mechanisms of natural transformation are similar among species, there is a broad variety of regulatory strategies [5]. The best studied competence regulators include alternative \(\sigma\) factors, transcription activators and transcriptional co-regulators. A well-studied alternative \(\sigma\) factor, \(\sigma^{\chi}\) of _S. pneumoniae_, is encoded by two identical _comX_ genes and is widely distributed among the Firmicutes. \(\sigma^{\chi}\) forms a complex with RNA polymerase that recognizes an eight bp sequence in the promoter of almost all _com_ genes that are required for transformation [21], resulting in the induction of competence. Similar mechanisms have been found in other streptococcal species and also in more distantly related phylogenetic lineages within the Firmicutes such as in the Lactobacillus. The most thoroughly studied example of direct transcriptional activation that drives competence development is found in _B. subtilis_. Here, competence development is induced when the ComK protein binds to its own promoter, creating an auto-stimulatory cycle that results in a ComK concentration increase and the development of competence [19; 22]. Only 15% of the population becomes competent, resulting in so-called bistability or phenotypic heterogeneity.