**1. Introduction**

Tuberculosis (TB), a prehistoric disease, remains one of the top 10 causes of death and the leading cause from a single infectious agent, *Mycobacterium tuberculosis*, despite global efforts in disease control programs during the past 20 years [1]. TB is a global disease, found in every country in the world [1]. It became mankind's oldest and worst enemy owing to its widespread nature across the world and developing resistance to known and available drugs [1]. In 2017, 10 million people developed TB, and

an estimated 1.3 million deaths among human immunodeficiency virus (HIV)-negative people and an additional 300,000 deaths from TB among HIV-positive people occurred [1]. The latest data from the Statistics South Africa show that TB is one of the top killers in South Africa [2], suggesting an urgent need to understand *M. tuberculosis* physiology to be able to come up with novel drugs and drug targets.

Despite living in the most advanced medicine era, TB remains a major threat to human health [1]. After 21 years of *M. tuberculosis* genome sequencing [3], to date its physiology is poorly understood and many proteins remain orphans. Genome sequencing analysis of *M. tuberculosis* H37Rv revealed the presence of 20 cytochrome P450 monooxygenases (CYPs/P450s) in its genome [3]. P450s are mixed function oxidoreductases ubiquitously distributed across the biological kingdoms [4]. P450s are well known for their role in essential cellular anabolic and catabolic processes.

Among 20 P450s, to date, the role of only six *M. tuberculosis* H37Rv P450s in its physiology have been elucidated [5]. CYP51B1, highly conserved P450 family across microbes, has been found to catalyse the 14α-demethylation of lanosterol [6–8]; CYP121A1 catalyses oxidative crosslinking of the two tyrosines in a cyclodipeptide [9]; CYP125A1 and CYP142A1 catalyse the 26-hydroxylation of cholesterol and cholest-4-en-3-one [10,11]; CYP124A1 catalyses the terminal hydroxylation of methyl-branched hydrocarbons such as those of phytanic acid and farnesol [12], cholesterol and related sterols [10,13], and vitamin D3 and CYP128A1 is involved in oxidation of menaquinone MK9 [14].

Among *M. tuberculosis* H37Rv P450s, the *CYP139A1* gene was found downstream of polyketide synthase genes (*pks10*, *pks7*, *pks8*, *pks17*, *pks9* and *pks11*) and situated next to macrolide transport protein [15,16]. Two of the polyketide synthases, *pks7* and *pks8*, were found to be essential for the survival of *M. tuberculosis* [17,18]. Polyketide synthases along with other genes were found to be part of biosynthetic gene clusters (BGCs). As per Medema et al. [19], a BGC can be defined as a physically clustered group of two or more genes in a particular genome that together encode a biosynthetic pathway for the production of a specialised metabolite (including its chemical variants). Bacteria, fungi and plants are known to possess different types of BGCs producing a variety of secondary metabolites that are beneficial to humans. Among the genes that are part of a BGC, P450s play a key role in contributing to the diversity of a secondary metabolite owing to their regio and stereo-specific oxidation [20]. Recently, comprehensive comparative analysis of P450s and those associated with secondary metabolism revealed a large number of P450s involved in the production of secondary metabolites in different bacterial species [21,22].

Based on *CYP139A1* location, this P450 is assumed to be involved in oxidative tailoring of the macrolide structure. In the latest study, involving comprehensive comparative analysis of P450s in bacterial species belonging to the genera *Mycobacterium* and *Streptomyces*, CYP139 P450s were found to be dominantly located in different secondary metabolite BGCs [22]. This strongly indicates that CYP139 P450s are possibly involved in the synthesis of secondary metabolites. This study is aimed at using an in silico approach to unravel the CYP139 P450 family's role in mycobacterial species physiology.
