Identification of the duplicated genes for S-adenosyl-l-methionine synthetase (metK1-sp and metK2-sp) in Streptomyces peucetius var. caesius ATCC 27952

AIMS

To characterize the function of both metK1-sp (sp1190) and metK2-sp (sp1566) in vitro and in vivo, and to study the regulation of doxorubicin production by overexpressing the metK.

METHODS AND RESULTS

We cloned two orfs into pET32a(+) respectively, and the formation of S-Adenosyl-l-methionine was clearly observed in the in vitro enzyme assays as functional MetKs. Reverse transcriptase polymerase chain reaction (PCR) analysis indicated that the transcripts for the metK1-sp were repressed as Streptomyces cells entered the decline phase, whereas that of the metK2-sp was induced, suggesting that these MetK proteins may be important for the growth and the regulation of secondary metabolites during the stationary growth phase, whether considered together or separately. Furthermore, we found that the introduction of high-copy-number plasmids containing the metK1-sp and metK2-sp resulted in 2.1- and 1.4-fold greater levels of doxorubicin production than the control transformants containing only the vector, respectively. We also attempted to disrupt the metK-sp and found that doxorubicin production from the metK1-sp-deleted mutant (Streptomyces peucetius/pNN1) was reduced when compared to the parent strain (S. peucetius var. caesius ATCC 27952).

CONCLUSIONS

The results of this study indicated that two metK are differentially expressed during cell growth, and that the expressions of the two metK genes are differentially regulated under the same conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Streptomyces peucetius var. caesius contains two genes, metK1-sp and metK2-sp, which encode functional S-adenosyl-l-methionine synthetase (MetK). The degree of homology (90% identity) found between the two genes shows that metK1-sp and metK2-sp are duplicated genes. Although there is currently no evidence for the relationship of the duplicated metK genes involved in the regulation of doxorubicin production, metK1-sp and metK2-sp may play a role in controlling the stimulation of antibiotic production during secondary metabolism.

Differential effects of the protein cofactor on the interactions between an RNase P ribozyme and its target mRNA substrate

RNase P from Escherichia coli is a tRNA-processing enzyme and consists of a catalytic RNA subunit (M1 RNA) and a protein component (C5 protein). M1GS, a gene-targeting ribozyme derived from M1, can cleave a herpes simplex virus 1 mRNA efficiently in vitro and inhibit its expression effectively in viral-infected cells. In this study, the effects of C5 on the interactions between a M1GS ribozyme and a model mRNA substrate were investigated by site-specific UV crosslink mapping. In the presence of the protein cofactor, the ribozyme regions crosslinked to the substrate sequence 3' immediately to the cleavage site were similar to those found in the absence of C5. Meanwhile, some of the ribozyme regions (e.g. P12 and J11/12) that were crosslinked to the leader sequence 5' immediately to the cleavage site in the presence of C5 were different from those regions (e.g. P3 and P4) found in the absence of the protein cofactor and were not among those that are believed to interact with a tRNA. Understanding how C5 affects the specific interactions between the ribozyme and its target mRNA may facilitate the development of gene-targeting ribozymes that function effectively in vivo, in the presence of cellular proteins.

Envelope glycoprotein determinants of increased fusogenicity in a pathogenic simian-human immunodeficiency virus (SHIV-KB9) passaged in vivo

Changes in the envelope glycoprotein ectodomains of a nonpathogenic simian-human immunodeficiency virus (SHIV-89.6) that was serially passaged in vivo have been shown to be responsible for the increased pathogenicity of the resulting virus, SHIV-KB9 (G. B. Karlsson, et al., J. Exp. Med. 188:1159-1171, 1998). The 12 amino acid changes in the envelope glycoprotein ectodomains resulted in increased chemokine receptor-binding and syncytium-forming abilities. Here we identify the envelope glycoprotein determinants of these properties. A single amino acid change in the gp120 third variable (V3) loop was both necessary and sufficient for the observed increase in the binding of the SHIV-KB9 gp120 glycoprotein to the CCR5 chemokine receptor. The increased syncytium-forming ability of SHIV-KB9 involved, in addition to the V3 loop change, changes in the second conserved (C2) region of gp120 (residue 225) and in the gp41 ectodomain (residues 564 and 567). The C2 and gp41 ectodomain changes influenced syncytium formation in a cooperative manner. Changes in the V1/V2 gp120 variable loops exerted a negative effect on syncytium formation and chemokine receptor binding, supporting a previously described role of these changes in immune evasion. The definition of the passage-associated changes that determine the efficiency of chemokine receptor binding and membrane fusogenicity will allow evaluation of the contribution of these properties to in vivo CD4-positive lymphocyte depletion.

RNase P ribozymes selected in vitro to cleave a viral mRNA effectively inhibit its expression in cell culture

An in vitro selection procedure was used to select RNase P ribozyme variants that efficiently cleaved the sequence of the mRNA encoding thymidine kinase of herpes simplex virus 1. Of the 45 selected variants sequenced, 25 ribozymes carried a common mutation at nucleotides 224 and 225 of RNase P catalytic RNA from Escherichia coli (G(224)G(225) --> AA). These selected ribozymes exhibited at least 10 times higher cleavage efficiency (k(cat)/K(m)) than that derived from the wild type ribozyme. Our results suggest that the mutated A(224)A(225) are in close proximity to the substrate and enhance substrate binding of the ribozyme. When these ribozyme variants were expressed in herpes simplex virus 1-infected cells, the levels of thymidine kinase mRNA and protein were reduced by 95-99%. Our study provides the first direct evidence that RNase P ribozyme variants isolated by the selection procedure can be used for the construction of gene-targeting ribozymes that are highly effective in tissue culture. These results demonstrate the potential for using RNase P ribozymes as gene-targeting agents against any mRNA sequences, and using the selection procedure as a general approach for the engineering of RNase P ribozymes.