Nucleosomes facilitate their own invasion

DNA wrapped in nucleosomes is sterically occluded, creating obstacles for polymerase, regulatory, remodeling, repair and recombination complexes, which require access to the wrapped DNA. How such complexes recognize and gain access to their DNA target sites is not known. Here we report the direct detection of a dynamic equilibrium conformational transition in nucleosomes that greatly increases the distance between the end of the nucleosomal DNA and the histone core. We quantified the equilibrium constant for this transition under physiological conditions. As predicted by these findings, addition of LexA protein to nucleosomes containing the LexA target site drives this conformational equilibrium toward the unwrapped, accessible state, simultaneously allowing stable LexA binding. This inherent property of nucleosomes allows any protein, whether an energy-dependent machine or a passive binder, to gain access even to buried stretches of nucleosomal DNA.     

Infants' knowledge of their own species

Recognition of individuals at first sight is important for social species and can be achieved by attending to facial or body information. Previous research suggests that infants possess a perceptual template for evolutionarily relevant stimuli, which may include humans, dangerous animals (e.g. snakes), but not non-dangerous animals. To be effective, such a mechanism should result in a systematic preference for attending to humans over non-dangerous animals. Using a preferential looking paradigm, the present studies investigated the nature of infants' early representation of humans. We show that 3.5- and six-month-old infants attend more to human beings than non-human primates (a gorilla or monkey) which are examplars of non-dangerous animals. This occurred when infants were presented with head or body information in isolation, as well as when both are presented simultaneously. This early preference for humans by 3.5 months of age suggests that there is a basic representation for humans, which includes both head and/or body information. However, neonates demonstrated a preference only for human faces over non-human primate faces, not for humans over non-human primates when the stimuli were presented with both head and body simultaneously. The results show that although neonates display a preference for human faces over others, preference for the human body only develops later, in the first few months of life. This suggests that infants have acquired some knowledge about the human body at 3.5 months of age that may have developed from their privileged experience with other humans in the first few months of life, rather than an innate ability to detect humans in their entirety.     

Chromosomes with a life of their own

B chromosomes (Bs) can be described as 'passengers in the genome', a term that has been used for the repetitive DNA which comprises the bulk of the genome in large genome species, except that Bs have a life of their own as independent chromosomes. As with retrotransposons they can accumulate in number, but in this case by various processes of mitotic or meiotic drive, based on their own autonomous ways of using spindles, especially in the gametophyte phase of the life cycle of flowering plants. This selfish property of drive ensures their survival and spread in natural populations, even against a gradient of harmful effects on the host plant phenotype. Bs are inhabitants of the nucleus and they are subject to control by 'genes' in the A chromosome (As) complement. This interaction with the As, together with the balance between drive and harmful effects makes a dynamic system in the life of a B chromosome, notwithstanding the fact that we are only now beginning to unravel the story in a few favoured species. In this review we concentrate mainly on recent developments in the Bs of rye and maize, two of the species currently receiving most attention. We focus on their population dynamics and on the molecular basis of their structural organisation and mechanisms of drive, as well as on their mode of origin and potential applications in plant biotechnology.     

Amplification of bacterial plasmids without blocking protein biosynthesis

The effect of amino acids (presence or absence from the growth media) and metal ions on the replication of Escherichia coli plasmids in rel A+ strains was studied. It was found that: (i) The absence of one amino acid from the growth media had no effect on the plasmid copy number in prototrophic E. coli strains: (ii) The presence of only one amino acid in artificial media free of amino acids had a negligible effect on the plasmid copy number for the amino acids Ala, Arg, Glu, His, Leu, Phe, Thr, Trp, and Tyr: (iii) The combination of Met and Thr caused a rise in pBR322 plasmid copy number up to 90-100 plasmid copies per cell: (iv) The Fe3+ concentration had an amplification effect on E. coli plasmids. The pBR322 plasmid copy number for media free of amino acids and supplemented with 0.2-0.4 mM FeCl3 was 60-80 plasmid copies per cell: (v) The combination of Fe3+ with certain amino acids (Ala, Arg, Glu, Leu, Thr, and Trp) leads to a dramatic increase in the plasmid copy number reaching 180-270 plasmid copies per cell for the plasmid pBR322 and 20-24 for the plasmid pR100.