Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTION(Stockmann & Piepersberg, 1992; Decker et al., 1995; Hyun et al., 2000) and it isbelieved that these two genes form a transcriptional unit due to the presence of thestop codon of dTDP-glucose synthase genes overlapping the start codon of the dTDP-D-glucose 4,6 dehydratase genes (Kirschning et al., 1998).In this study the strD gene was targeted as it is commonly found in aminoglycosidedesoxysugar producing actinobacteria (Stockman & Piepersberg, 1992). The strDgene exhibits 60 – 70 % amino acid identities with corresponding genes inactinobacteria (Gaisser et al., 1997; Stratmann et al., 1999). Additional targets couldhave also included strE, strM and strL genes which are also common in theproduction of 6-deoxyhexose constituents in aminoglycoside antibiotics (Stockmann& Piepersberg, 1992). An interesting aspect of the strE gene is that it has been shownto be more closely related between actinobacterial species than to dehydratases fromspecies of other orders (Decker et al., 1996). The strE gene found in actinobacteriahave been shown to contain between 55 – 65 % amino acid identities (Gaisser et al.,1997; Stratmann et al., 1999).1.3.3 β-lactam AntibioticsThe discovery made by Alexander Fleming in 1929, that Penicillium culturesproduced an antibiotic known as penicillin was the first microbial SM developed as achemotherapeutic agent (Rolinson, 1995). Penicillins belong to the β-lactam class ofnaturally occurring antibiotics produced by microorganisms. Fungal speciessynthesize the majority of β-lactams (Brakhage,1998). However, actinobacteria areprolific producers of certain classes of β-lactams these include the clavams,cephamycins, carabapenams and monobactams. β-lactams are classified into fivegroups (Table 5). All are produced by a similar biosynthetic pathway and contain ageneral structure of a 4-membered β-lactam ring (Figure 11).Figure 11. The general structure of β-lactams (adapted from Demain & Elander,1999)._____________________________________________________________________21