The molecular mechanism of Drosophila restricting telomeric transposons.

Linear chromosomes of eukaryotes are protected by a DNA-protein-RNA structure called telomere. Remarkably and unlike those of most organisms studied, Drosophila telomeric DNA is not composed of a group of short repeats, but three classes of retrotransposons at the chromosome ends. Telomeric transposons in Drosophila on the other hand serves the function of elongating the host chromosomes yet prevent little harm to the host genome as their insertion sites are strictly limited to the telomere. How the Drosophila host achieves such precise regulation is still unclear. The currently known genome-wide repression of transposon expression includes piRNA pathway and the heterochromatin pathway involving H3K9me3. Recent studies have found that Drosophila telomere capping proteins are involved in the specific regulation of telomeric retrotransposons. In this review, we discuss the specific functions of telomere capping proteins in regulating telomeric transposons. By studying how the Drosophila host interacts and regulates telomeric transposons, we hope to shed lights on universal principles in guiding their co-evolution.

Validation of the intact zwittermicin A biosynthetic gene cluster and discovery of a complementary resistance mechanism in Bacillus thuringiensis.

Zwittermicin A (ZmA) is a hybrid polyketide-nonribosomal peptide produced by certain Bacillus cereus group strains. It displays broad-spectrum antimicrobial activity. Its biosynthetic pathway in B. cereus has been proposed through analysis of the nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) modules involved in ZmA biosynthesis. In this study, we constructed a bacterial artificial chromosome (BAC) library from Bacillus thuringiensis subsp. kurstaki strain YBT-1520 genomic DNA. The presence of known genes involved in the biosynthesis of ZmA in this BAC library was investigated by PCR techniques. Nine positive clones were identified, two of which (covering an approximately 60-kb region) could confer ZmA biosynthesis ability upon B. thuringiensis BMB171 after simultaneous transfer into this host by two compatible shuttle BAC vectors. Another previously unidentified gene cluster, named zmaWXY, was found to improve the yield of ZmA and was experimentally defined to function as a ZmA resistance transporter which expels ZmA from the cells. Putative transposase genes were detected on the flanking regions of the two gene clusters (the ZmA synthetic cluster and zmaWXY), which suggests a mobile nature of these two gene clusters. The intact ZmA gene cluster was validated, and a resistance mechanism complementary to that for zmaR (the previously identified ZmA self-resistance gene) was revealed. This study also provided a straightforward strategy to isolate and identify a huge gene cluster from Bacillus.