Effects of plant species richness on invasion dynamics, disease outbreaks, insect abundances and diversity.

Declining biodiversity represents one of the most dramatic and irreversible aspects of anthropogenic global change, yet the ecological implications of this change are poorly understood. Recent studies have shown that biodiversity loss of basal species, such as autotrophs or plants, affects fundamental ecosystem processes such as nutrient dynamics and autotrophic production. Ecological theory predicts that changes induced by the loss of biodiversity at the base of an ecosystem should impact the entire system. Here we show that experimental reductions in grassland plant richness increase ecosystem vulnerability to invasions by plant species, enhance the spread of plant fungal diseases, and alter the richness and structure of insect communities. These results suggest that the loss of basal species may have profound effects on the integrity and functioning of ecosystems.

Nucleotide preferences in sequence-specific recognition of DNA by c-myb protein.

Using a binding site selection procedure, we have found that sequence-specific DNA-binding by the mouse c-myb protein involves recognition of nucleotides outside of the previously identified hexanucleotide motif. Oligonucleotides containing a random nucleotide core were immunoprecipitated in association with c-Myb, amplified by the Polymerase Chain Reaction and cloned in plasmids prior to sequencing. By alignment of sequences it was apparent that additional preferences existed at each of three bases immediately 5' of the hexanucleotide consensus, allowing an extension of the preferred binding site to YGRCVGTTR. The contributions of these 5' nucleotides to binding affinity was established in bandshift analyses with oligonucleotides containing single base substitutions; in particular, it was found that replacement of the preferred guanine at position -2 with any other base greatly reduced c-Myb binding. We found that the protein encoded by the related B-myb gene bound the preferred c-Myb site with similar affinity; however, B-Myb and c-Myb showed distinct preferences for the identity of the nucleotide at position -1 relative to the hexanucleotide consensus. This study demonstrates that the c-Myb DNA-binding site is more extensive than recognised hitherto and points to similar but distinct nucleotide preferences in recognition of DNA by related Myb proteins.

Characterization of the sequence-specific interaction of mouse c-myb protein with DNA.

We have examined parameters that affect sequence-specific interactions of the mouse c-myb protein with DNA oligomers containing the Myb-binding motif (CA/CGTTPu). Complexes formed between these oligomers and in vitro translated c-myb proteins were analysed by electrophoresis on non-denaturing polyacrylamide gels using the mobility-shift assay. By progressive truncation of c-myb coding sequences it was demonstrated that amino acids downstream of a region of three imperfect 51-52 residue repeats (designated R1, R2 and R3), which are located close to the amino terminus of the protein, had no qualitative or quantitative effect on the ability to interact specifically with this DNA motif. However, removal of only five amino acids of the R3 repeat completely abolished this activity. The contribution of individual DNA-binding domain repeats to this interaction was investigated by precisely deleting each individually: it was demonstrated that a combination of R2 and R3 was absolutely required for complex formation while the R1 repeat was completely dispensible. c-myb proteins showed quantitatively greater interaction with oligomers containing duplicated rather than single Myb-binding motif, in particular where these were arranged in tandem. Moreover, it was observed that c-myb protein interacted with these tandem motifs as a monomer. These findings imply that a single protein subunit straddles adjacent binding sites and the implications for c-myb activity are discussed.

The Drosophila hairy protein acts in both segmentation and bristle patterning and shows homology to N-myc.

The Drosophila segmentation gene, hairy (h), acts to regulate embryonic segmentation and bristle pattern. We present the DNA sequence of the h gene and of h cDNAs, thereby deducing the organization of the h transcripts. The h gene encodes a 337 amino acid protein that acts in both embryonic segmentation and adult bristle patterning. The h protein includes a domain that shows extensive similarity to a domain of the proto-oncogene N-myc that may be involved in DNA binding and/or protein dimerization. We discuss mechanisms of h action as a transcriptional regulator.

The induction of Friend erythroleukaemia differentiation is markedly affected by expression of a transfected c-myb cDNA.

A minigene containing a cDNA encoding the normal mouse p80c-myb protein under strong promoter control was used to transfect Friend erythroleukaemia cells. Expression of the transfected gene was found to result in elevated levels of p80c-myb which were not subject to rapid down-modulation by inducers of Friend cell differentiation as was the product of the endogenous c-myb gene. Continued synthesis of p80c-myb was found to be associated with a marked decrease in differentiation of Friend cells and we concluded that normal down-regulation of c-myb expression may be a necessary event in differentiation of these cells.

Evidence that rifampicin can stimulate readthrough of transcriptional terminators in Escherichia coli, including the attenuator of the rpoBC operon.

The genes encoding the beta and beta' subunits of RNA polymerase in E.coli, rpoB and rpoC, lie downstream of at least two ribosomal protein genes, rplJ (encoding L10) and rplL (L7/12), in a common operon. All four genes are served by promoter PL10, and an attenuator (partial terminator) of transcription, t1, lies between rplJL and rpoBC. Treatment of E.coli with rifampicin, under conditions producing partial inhibition of general RNA synthesis, can stimulate transcription of rpoBC. We have investigated the locus of this effect by fusing PL10 and t1 separately to galK, in suitable plasmids. Our studies of these fusions, and similar fusions involving transcriptional terminators derived from coliphage T7, indicate that low concentrations of rifampicin cause increased readthrough of several different transcriptional terminators in E.coli in vivo, including rpo t1. We discuss whether or not this unspecific mechanism is solely responsible for the observed stimulatory effects of the drug on rpoBC transcription.

Effect of rifampicin on expression of lacZ fused to promoters or terminators of the E.coli rpoBC operon.

The genes encoding the beta and beta' subunits of RNA polymerase in E.coli lie downstream of at least two ribosomal protein genes in a single unit of transcription. Treatment of E.coli with rifampicin, under conditions producing partial inhibition of general RNA synthesis, can strongly stimulate transcription of the polymerase genes, while activating the neighbouring ribosomal genes only slightly. We have investigated the mechanism of this effect by fusing strong promoters, a weak internal promoter, and an attenuator of the polymerase operon to the lacZ gene, in derivatives of plasmid pMC81. Studies of these fusions confirm our conclusion, based on similar fusions to galK, that rifampicin can foster readthrough of transcriptional terminators. They also suggest the existence of extra terminators and anti-termination elements in the above transcription unit.