Expression of the SalI restriction-modification system of Streptomyces albus G in Escherichia coli

Towards a new science of secondary metabolism

Secondary metabolites are a reliable and very important source of medicinal compounds. While these molecules have been mined extensively, genome sequencing has suggested that there is a great deal of chemical diversity and bioactivity that remains to be discovered and characterized. A central challenge to the field is that many of the novel or poorly understood molecules are expressed at low levels in the laboratory-such molecules are often described as the 'cryptic' secondary metabolites. In this review, we will discuss evidence that research in this field has provided us with sufficient knowledge and tools to express and purify any secondary metabolite of interest. We will describe 'unselective' strategies that bring about global changes in secondary metabolite output as well as 'selective' strategies where a specific biosynthetic gene cluster of interest is manipulated to enhance the yield of a single product.

Restriction-modification gene complexes as selfish gene entities: roles of a regulatory system in their establishment, maintenance, and apoptotic mutual exclusion

We have reported some type II restriction-modification (RM) gene complexes on plasmids resist displacement by an incompatible plasmid through postsegregational host killing. Such selfish behavior may have contributed to the spread and maintenance of RM systems. Here we analyze the role of regulatory genes (C), often found linked to RM gene complexes, in their interaction with the host and the other RM gene complexes. We identified the C gene of EcoRV as a positive regulator of restriction. A C mutation eliminated postsegregational killing by EcoRV. The C system has been proposed to allow establishment of RM systems in new hosts by delaying the appearance of restriction activity. Consistent with this proposal, bacteria preexpressing ecoRVC were transformed at a reduced efficiency by plasmids carrying the EcoRV RM gene complex. Cells carrying the BamHI RM gene complex were transformed at a reduced efficiency by a plasmid carrying a PvuII RM gene complex, which shares the same C specificity. The reduction most likely was caused by chromosome cleavage at unmodified PvuII sites by prematurely expressed PvuII restriction enzyme. Therefore, association of the C genes of the same specificity with RM gene complexes of different sequence specificities can confer on a resident RM gene complex the capacity to abort establishment of a second, incoming RM gene complex. This phenomenon, termed "apoptotic mutual exclusion," is reminiscent of suicidal defense against virus infection programmed by other selfish elements. pvuIIC and bamHIC genes define one incompatibility group of exclusion whereas ecoRVC gene defines another.

Characterization of a XhoI isoschizomer in Streptomyces aureofaciens after actinophage infection

After infection of tetracycline producing strains of S. aureofaciens with actinophages mu 1/6 and B1 some phage resistant colonies were obtained in each experiment. These colonies expressed a new restriction-modification (RM) system of type II, which was different from the common RM system (SauLPI) of these strains recognizing the sequence GCCGGC. This new RM system was not detected before in parental strains. The new endonuclease was purified from a phage resistant strain of S. aureofaciens B96, using two step column chromatography to the grade without non-specific nucleolytic activity. SauLPII endonuclease recognized and cleaved the palindromic hexanucleotide sequence 5'-C/TCGAG-3', thus it was a true isoschizomer of XhoI.