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Muhammad Sarwar Khan

Publications and source records attributed to Muhammad Sarwar Khan.

7 recordsLinked to original sources
PubMed2006

Hybrid transcription-mediated transgene regulation in plastids.

Plastids are semi-autonomous plant organelles with their own transcription and translation machinery that are suitable for producing high levels of transgenic proteins with a bona fide structure. However, the transcription of endogenous plastid genes is largely dependent on nuclear-encoded transcription factors, and the identification of these factors has implications for transgene regulation. A novel transcription system has recently been reported that works independently of the plastid transcription machinery and is potentially useful for transgene containment and to express genes with the assurance of high-level protein accumulation.

Gene Expression Regulation, Plant↗
PubMed2005

Unraveling the complexities of plastid transcription in plants.

Two RNA polymerases, plastid-encoded PEP and nuclear-encoded NEP, serve different sets of genes in plastids but share the transcription of housekeeping genes. Thus far, three genes have been identified that encode NEP -RpoTp, RpoTmp and RpoTm. The gene products of RpoTp and RpoTm are targeted to plastids and mitochondria, respectively. However, the RpoTmp was regarded as a dual targeting polymerase, although recent evidence demonstrates that the enzyme targets exclusively to mitochondria. This experimental discrepancy has important implications and changes our present understanding of the plastid transcription machinery.

DNA-Directed RNA Polymerases↗
PubMed2005

Intein-mediated protein trans-splicing and transgene containment in plastids.

Transgenes in plastids are contained by stringent maternal inheritance in most cultivated plant species and their expression yields high levels of protein with bona fide structure. Nevertheless, transfer of plastid genes to the nucleus has been reported, with implications for transgene containment. The significance of these transfers will depend on the likelihood that they will become functional nuclear genes. Recently a novel approach, intein-mediated protein trans-splicing, has been demonstrated promising to yield transgenic plants with greatly reduced risk of genetic outcrossing.

Inteins↗
PubMed2005

Phage phiC31 integrase: a new tool in plastid genome engineering.

Plastid transformation offers the unique advantages of high-level transgene expression and increased transgene containment compared with conventional transgenic technologies. The process relies on the homologous recombination machinery of the plastid incorporating foreign DNA into the plastome, which restricts the method to species where this type of incorporation works well. However, Pal Maliga and colleagues have recently reported a novel approach for integrating foreign DNA into the plastid genome that works independently of homologous recombination.

Bacteriophages↗
PubMed2004

Metabolic engineering of the chloroplast genome using the Echerichia coli ubiC gene reveals that chorismate is a readily abundant plant precursor for p-hydroxybenzoic acid biosynthesis.

p-Hydroxybenzoic acid (pHBA) is the major monomer in liquid crystal polymers. In this study, the Escherichia coli ubiC gene that codes for chorismate pyruvate-lyase (CPL) was integrated into the tobacco (Nicotiana tabacum) chloroplast genome under the control of the light-regulated psbA 5' untranslated region. CPL catalyzes the direct conversion of chorismate, an important branch point intermediate in the shikimate pathway that is exclusively synthesized in plastids, to pHBA and pyruvate. The leaf content of pHBA glucose conjugates in fully mature T1 plants exposed to continuous light (total pooled material) varied between 13% and 18% dry weight, while the oldest leaves had levels as high as 26.5% dry weight. The latter value is 50-fold higher than the best value reported for nuclear-transformed tobacco plants expressing a chloroplast-targeted version of CPL. Despite the massive diversion of chorismate to pHBA, the plastid-transformed plants and control plants were indistinguishable. The highest CPL enzyme activity in pooled leaf material from adult T1 plants was 50,783 pkat/mg of protein, which is equivalent to approximately 35% of the total soluble protein and approximately 250 times higher than the highest reported value for nuclear transformation. These experiments demonstrate that the current limitation for pHBA production in nuclear-transformed plants is CPL enzyme activity, and that the process becomes substrate-limited only when the enzyme is present at very high levels in the compartment of interest, such as the case with plastid transformation. Integration of CPL into the chloroplast genome provides a dramatic demonstration of the high-flux potential of the shikimate pathway for chorismate biosynthesis, and could prove to be a cost-effective route to pHBA. Moreover, exploiting this strategy to create an artificial metabolic sink for chorismate could provide new insight on regulation of the plant shikimate pathway and its complex interactions with downstream branches of secondary metabolism, which is currently poorly understood.

Chloroplasts↗
PubMed2004

Taming plastids for a green future.

Plant genetic engineering will probably contribute to the required continued increase in agricultural productivity during the coming decades, and moreover, plants can potentially provide inexpensive production platforms for pharmaceuticals and nutraceuticals. With the advent of technologies for altering the genetic information inside chloroplasts, a new attractive target for genetic engineering has become available to biotechnologists. Potential advantages over conventional nuclear transformation include high transgene expression levels and increased biosafety because of maternal organelle inheritance in most crops. This review summarizes the state of the art in chloroplast genetic engineering and describes how reverse genetics approaches enhance our understanding of photosynthesis and other important chloroplast functions. Furthermore, promising strategies by which chloroplast genetic engineering might contribute to the successful modification of plant metabolism are discussed.

Chloroplasts↗