Translating biomolecular recognition into nanomechanics.

We report the specific transduction, via surface stress changes, of DNA hybridization and receptor-ligand binding into a direct nanomechanical response of microfabricated cantilevers. Cantilevers in an array were functionalized with a selection of biomolecules. The differential deflection of the cantilevers was found to provide a true molecular recognition signal despite large nonspecific responses of individual cantilevers. Hybridization of complementary oligonucleotides shows that a single base mismatch between two 12-mer oligonucleotides is clearly detectable. Similar experiments on protein A-immunoglobulin interactions demonstrate the wide-ranging applicability of nanomechanical transduction to detect biomolecular recognition.

Direct detection of a BRAF mutation in total RNA from melanoma cells using cantilever arrays.

Malignant melanoma, the deadliest form of skin cancer, is characterized by a predominant mutation in the BRAF gene. Drugs that target tumours carrying this mutation have recently entered the clinic. Accordingly, patients are routinely screened for mutations in this gene to determine whether they can benefit from this type of treatment. The current gold standard for mutation screening uses real-time polymerase chain reaction and sequencing methods. Here we show that an assay based on microcantilever arrays can detect the mutation nanomechanically without amplification in total RNA samples isolated from melanoma cells. The assay is based on a BRAF-specific oligonucleotide probe. We detected mutant BRAF at a concentration of 500 pM in a 50-fold excess of the wild-type sequence. The method was able to distinguish melanoma cells carrying the mutation from wild-type cells using as little as 20 ng µl(-1) of RNA material, without prior PCR amplification and use of labels.

Rapid and label-free nanomechanical detection of biomarker transcripts in human RNA.

The availability of entire genome sequences has triggered the development of microarrays for clinical diagnostics that measure the expression levels of specific genes. Methods that involve labelling can achieve picomolar detection sensitivity, but they are costly, labour-intensive and time-consuming. Moreover, target amplification or biochemical labelling can influence the original signal. We have improved the biosensitivity of label-free cantilever-array sensors by orders of magnitude to detect mRNA biomarker candidates in total cellular RNA. Differential gene expression of the gene 1-8U, a potential marker for cancer progression or viral infections, has been observed in a complex background. The measurements provide results within minutes at the picomolar level without target amplification, and are sensitive to base mismatches. This qualifies the technology as a rapid method to validate biomarkers that reveal disease risk, disease progression or therapy response. We foresee cantilever arrays being used as a tool to evaluate treatment response efficacy for personalized medical diagnostics.

The relationship between carotenoid biosynthesis and the assembly of the light-harvesting LH2 complex in Rhodobacter sphaeroides.

Coloured carotenoids play some undefined role in the assembly of a functional light-harvesting 2 (LH2) complex in photosynthetic bacteria. We have used a series of transposon Tn5 insertion mutants disrupted at various stages of the carotenoid-biosynthetic pathway, together with an LH2 deletion/insertion mutant, to investigate this effect in Rhodobacter sphaeroides. Mutants were initially characterized by low-temperature absorbance spectroscopy and ultrastructural analysis: Northern-blot analysis demonstrated normal pucBA transcripts for LH2 polypeptides in all the carotenoid mutants. Analysis of translation of the puc transcript and investigation of the fate of any resulting LH2 polypeptides by SDS/PAGE, Western-blot and pulse-chase experiments clearly demonstrated that, in the absence of coloured carotenoids, the LH2 alpha- and beta-polypeptides are synthesized but are rapidly turned over and do not become stably integrated into the membrane. Complementation of mutants with lesions in the crtB and crtI genes, encoding phytoene synthase and phytoene desaturase respectively, with the cloned R. sphaeroides crtI gene, resulted in restoration of carotenoid biosynthesis and stable assembly of the LH2 complex in the crtI mutant but not in the crtB mutant, despite the presence of the CrtI protein.

Early steps in carotenoid biosynthesis: sequences and transcriptional analysis of the crtI and crtB genes of Rhodobacter sphaeroides and overexpression and reactivation of crtI in Escherichia coli and R. sphaeroides.

In the purple photosynthetic bacterium Rhodobacter sphaeroides, the desaturation of phytoene has already been implicated in the assembly of the light-harvesting 2 complex (H.P. Lang and C.N. Hunter, Biochem. J. 298:197-205, 1994). The phytoene synthase and desaturase enzymes mediate the first steps specific for carotenoid biosynthesis up to and including the synthesis of the colored carotenoid neurosporene. In this report, we present the DNA and deduced amino acid sequences of the genes encoding these proteins, namely, crtB and crtI, from R. sphaeroides and present evidence for the existence of a crtIB operon. Both genes have been shown to possess putative puc and puf operon-like promoter sequences, and oxygen regulation and the point of initiation of the crtI transcript have been demonstrated. The complete crtI gene has been overexpressed in Escherichia coli and R. sphaeroides and shown to catalyze three desaturations of phytoene to give neurosporene. This activity was shown to be ATP dependent, and the cofactor requirement was investigated by using a spectroscopic assay for in vitro carotenogenic activity. Although the crtI and crtB genes have been sequenced from a number of different organisms, the transcriptional organization and regulation of these genes have not been analyzed in detail. In this report, we have located the transcription initiation point and have shown that R. sphaeroides possesses an oxygen-regulated CrtI-type phytoene desaturase gene that forms a transcriptional operon with crtB.

Complete DNA sequence, specific Tn5 insertion map, and gene assignment of the carotenoid biosynthesis pathway of Rhodobacter sphaeroides.

The carotenoid biosynthesis genes form a cluster within the genome of Rhodobacter sphaeroides, lying in the middle of a larger cluster and 45 kb in length, which contains genes for bacteriochlorophyll biosynthesis and for the reaction center and light-harvesting apoproteins. The positions and approximate limits of the carotenoid genes were determined previously by localized transposon Tn5 mutagenesis and by comparison with the closely related Rhodobacter capsulatus carotenoid gene cluster. In this report, analysis of the DNA and deduced amino acid sequences of the carotenoid genes in R. sphaeroides are presented. Twenty-five Tn5 insertion mutants were used to produce a base-specific Tn5 insertion map of this region, and carotenoid gene assignment was supported by spectroscopic, ultrastructural, and high-pressure liquid chromatography analyses of these mutants. A region in the 3' end of crtD which affects bacteriochlorophyll biosynthesis was discovered, and CrtA was found to possess a proline-rich C-terminal region containing a repeated (Ala-Pro)n motif. CrtF also showed a high degree of sequence conservation with eukaryotic O-methyltransferases. This study provides gene sequences and assignments based upon a comprehensive structural, spectroscopic, and biochemical analysis of a range of carotenoid biosynthetic mutants; in each mutation, the point of Tn5 insertion is determined accurate to 1 bp on the gene cluster.

Introduction of new carotenoids into the bacterial photosynthetic apparatus by combining the carotenoid biosynthetic pathways of Erwinia herbicola and Rhodobacter sphaeroides.

Carotenoids have two major functions in bacterial photosynthesis, photoprotection and accessory light harvesting. The genes encoding many carotenoid biosynthetic pathways have now been mapped and cloned in several different species, and the availability of cloned genes which encode the biosynthesis of carotenoids not found in the photosynthetic genus Rhodobacter opens up the possibility of introducing a wider range of foreign carotenoids into the bacterial photosynthetic apparatus than would normally be available by producing mutants of the native biosynthetic pathway. For example, the crt genes from Erwinia herbicola, a gram-negative nonphotosynthetic bacterium which produces carotenoids in the sequence of phytoene, lycopene, beta-carotene, beta-cryptoxanthin, zeaxanthin, and zeaxanthin glucosides, are clustered within a 12.8-kb region and have been mapped and partially sequenced. In this paper, part of the E. herbicola crt cluster has been excised and expressed in various crt strains of Rhodobacter sphaeroides. This has produced light-harvesting complexes with a novel carotenoid composition, in which the foreign carotenoids such as beta-carotene function successfully in light harvesting. The outcome of the combination of the crt genes in R. sphaeroides with those from E. herbicola has, in some cases, resulted in an interesting rerouting of the expected biosynthetic sequence, which has also provided insights into how the various enzymes of the carotenoid biosynthetic pathway might interact. Clearly this approach has considerable potential for studies on the control and organization of carotenoid biosynthesis, as well as providing novel pigment-protein complexes for functional studies.