Molecular Cloning and Heterologous Expression of a Biosynthetic Gene Cluster for the Antitubercular Agent d -Cycloserine Produced by Streptomyces lavendulae

Abstract

In the present study, we successfully cloned a 21-kb DNA fragment containing a D-cycloserine (DCS) biosynthetic gene cluster from a DCS-producing Streptomyces lavendulae strain, ATCC 11924. The putative gene cluster consists of 10 open reading frames (ORFs), designated dcsA to dcsJ. This cluster includes two ORFs encoding D-alanyl-D-alanine ligase (dcsI) and a putative membrane protein (dcsJ) as the self-resistance determinants of the producer organism, indicated by our previous work. When the 10 ORFs were introduced into DCS-nonproducing Streptomyces lividans 66 as a heterologous host cell, the transformant acquired DCS productivity. This reveals that the introduced genes are responsible for the biosynthesis of DCS. As anticipated, the disruption of dcsG, seen in the DCS biosynthetic gene cluster, made it possible for the strain ATCC 11924 to lose its DCS production. We here propose the DCS biosynthetic pathway. The soil-dwelling genus Streptomyces undergoes a complex morphological differentiation and produces an enormous variety of bioactive secondary metabolites. Because they include clinically useful antibiotics and immunosuppressants, the genus Streptomyces occupies an important position as an industrial microorganism.D-Cycloserine (DCS), a cyclic structural analogue of D-alanine, is produced by "Streptomyces garyphalus" and Streptomyces lavendulae. This antibiotic is used as an antitubercular agent (21). Since DCS is similar to D-alanine, it prevents the action of both alanine racemase and D-alanyl-D-alanine ligase, which are necessary for the biosynthesis of a bacterial cell wall. Thus, DCS functions as an inhibitor of bacterial cell wall biosynthesis (16,19). Although the structure of DCS is very simple, the biosynthetic genes for DCS have never been cloned until now.In general, antibiotic biosynthetic genes form a cluster and are adjacent to their self-resistance genes. We have previously cloned a gene (orfB) that confers resistance to DCS on Streptomyces lividans and Escherichia coli from DCS-producing S. garyphalus (CSH) 5-12 (17). The sequence analysis suggests that the gene may encode a membrane protein that is necessary for the excretion of the DCS outside the cell. We have found that the same gene is also present in S. lavendulae ATCC 25233 (20). We have also previously demonstrated that Dalanyl-D-alanine ligase, which is a target enzyme of DCS, functions as a self-resistance determinant in S. lavendulae ATCC 25233 (20). Interestingly, the gene encoding D-alanyl-D-alanine ligase, designated ddlS, was located just upstream of the putative membrane protein gene, orfB (20). Although the selfresistance genes in the DCS producers have been thoroughly analyzed, no attempt to clone the DCS biosynthetic genes has been carried out yet.To clone the biosynthetic genes for DCS from another DCSproducing S. lavendulae strain, ATCC 11924, we first investigated whether this strain harbors orfB and ddlS. Since both genes were found to be conserved, the flanking region was cloned from the chromosomal DNA. Here, we ...

Crystallographic Study To Determine the Substrate Specificity of an l -Serine-Acetylating Enzyme Found in the d -Cycloserine Biosynthetic Pathway

“…Escherichia coli BL21(DE3)pLysS transformed with the plasmid was used to obtain the recombinant DcsE protein. Expression and purification of the recombinant DcsE were conducted as described previously (10).…”

“…1. Some of the enzymes catalyzing each reaction have already been identified by our group (10) and another (16). In the first biosynthetic step, the acetylation of L-serine by acetyl coenzyme A (acetyl-CoA)-dependent DcsE is necessary.…”

“…The cluster has eight genes (dcsA to dcsH) that would be responsible for DCS biosynthesis and two genes (dcsI and dcsJ) as self-resistance determinants for the producer organism (11,12). Based on our studies (10) and those of another group (13)(14)(15), we have proposed a putative DCS biosynthetic pathway in S. lavendulae ATCC 11924, as shown in Fig. 1.…”

“…We have recently reported the cloning and heterologous expression of a biosynthetic gene cluster for DCS from DCS-producing S. lavendulae ATCC 11924 (10). The cluster has eight genes (dcsA to dcsH) that would be responsible for DCS biosynthesis and two genes (dcsI and dcsJ) as self-resistance determinants for the producer organism (11,12).…”

“…In general, O-acetyl-L-serine, which is generated from L-serine in the presence of acetyl-CoA by L-serine O-acetyltransferase (SAT), is an intermediate for the biosynthesis of L-cysteine (18). Interestingly, DcsE, which displays no homology to well-known SATs but high homology to L-homoserine O-acetyltransferases (HATs) in the L-methionine biosynthetic pathway (19,20), acetylates the hydroxyl group of L-serine (10). A previous report (21) showed that a putative HAT gene product found in Aspergillus nidulans, which shows 31% sequence identity with DcsE, is involved in L-cysteine biosynthesis; however, the enzymatic function of the gene product has not yet been clarified.…”