Heterochromatin is a prevalent chromatin state among eukaryotes that has critical functions in chromosome segregation, control of genomic stability and epigenetic regulation of gene expression. Recent studies (for a review see: Vavasseur et al, 2008) conducted in the fission yeast Schizosaccharomyces pombe, reveal that two RNAi complexes, the RNAi-induced transcriptional gene silencing (RITS) complex and the RNA-directed RNA polymerase complex (RDRC), are part of a RNAi machinery involved in the initiation, propagation and maintenance of heterochromatin assembly. It appears that these two complexes localize in a siRNA-dependent manner on chromosomes, at the site of heterochromatin assembly. Moreover, these studies reveal an unprecedented and central role for RNA polymerase II (RNApII) in RNAi-dependent heterochromatin assembly. RNApII synthesizes a nascent transcript that is believed to serve as a RNA platform to recruit, RITS, RDRC and possibly other complexes required for heterochromatin assembly. Recent findings indicate that RNAi as well as an exosome-dependent RNA degradation process contribute to heterochromatic gene silencing. These findings challenge the widely accepted view that heterochromatic gene silencing is caused strictly by chromatin compaction. RNAi-dependent chromatin modifications have been observed throughout the eukaryotic kingdom the mechanisms considered here may occur in a large range of eukaryotes.

Further reading:

Vavasseur et al, 2008. Heterochromatin Assembly and Transcriptional Gene Silencing under the Control of Nuclear RNAi: Lessons from Fission Yeast. In: RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity. Ed, Morris K.V., Caister Academic Press, Norfolk, UK

Tost, J. 2008 Epigenetics Caister Academic Press, Norfolk, UK

MicroRNA (miRNA) is a small RNA (~22 nucleotides). The miRNA genes have been discovered in plants, invertebrates, and vertebrates. These miRNAs can regulate gene expression to inhibit translation of target messenger RNAs (mRNAs), and sometimes direct many rounds of site-specific mRNA cleavage in mammalian cells. Beyond the cutting-edged criteria of RNA interference (RNAi) by complementarily pairing of short interfering RNA (siRNA), the miRNAs, which were incomplementarily paired, are also encoded by several viruses, such as herpesviruses and human immunodeficiency virus type 1 (HIV-1). Intriguingly, gene expression of HIV-1 genome has been recently shown to be epigenetically regulated via a novel process of transcriptional repression by the miRNA of virus itself. However, role of the viral miRNA for gene regulation is not still well understood mechanistically as compared with host miRNAs.

A recently published paper (Fujii and Saksena, Chapter 7) describes the target prediction of a viral miRNA, miR-N367 and the conservation of secondary structure of pre-miR-N367 into mir-98/let-7 and mir-181a-2 in human miRNAs whose targets in HIV-1 genome could be related to HIV-1 transcriptional system. Also demonstrated is that the sequences of viral nef si/miRNA are conserved in plant miRNAs and the nef si/miRNAs was enabled to express in Arabidopsis thaliana similar to human cell. It is hypothesized that the orphaned non-selfish miRNAs may evolve and jump on to other RNAs, which can transposably lead to spread of these miRNAs from some plant and vertebrate genomes through feeding of miRNAs-containing foods, viruses, etc. Equivocally, miRNAs can be picked up into the lentiviral transposon, such as HIV-1. Therefore, the viral miR-N367 would necessarily be a HIV-1 silencer to be inclusively incorporated into HIV-1 itself. The virulence may be lost with recombination and mutation by miRNAs from the viral and host genome. The question is, whether the encoded miRNAs are mediating the viral and host genomic evolution.

Further reading:

RNA and the Regulation of Gene Expression

Epigenetics

More than 30 years have passed since Legionella pneumophila, the causative agent of Legionnaires' disease, was identified as a new human pathogen. First recognized due to the epidemic of pneumonia that followed the 1976 Legionnaires' convention in Philadelphia, USA, legionellosis is still a disease of medical and public interest. Legionella is commonly found in aquatic habitats, where its ability to survive and to multiply within different protozoa equips the bacterium to be transmissible and pathogenic to humans. However, these very traits also make Legionella a favoured model system to analyse the mechanisms by which bacteria survive, acquire nutrients, and replicate within macrophages and the lung.

With the application of modern molecular genetic and cell biological techniques, we have begun to understand the mechanisms used by L. pneumophila to multiply within protozoa and alveolar macrophages. Also, we have gained insight into the specific regulatory cascades that govern cell cycle-dependent differentiation as well as the general mechanisms of gene regulation in L. pneumophila.

Thanks to the recent publication of the genome sequences of four L. pneumophila strains, it is now feasible to investigate the whole genome in silico, the transcriptome via microarrays, and the proteome by two-dimensional gel electrophoresis. Thus, new questions can be asked and impressive amounts of data can be generated and evaluated. Furthermore, research in the fields of clinical features, diagnosis, treatment and epidemiology continues to generate new strategies for management and prevention of disease and more questions for basic scientists.

A newly published book on Legionella covers a wide range of topics from the history of the identification of Legionella and clinical disease treatment, to the microbe’s gene expression and secretion systems as well as its strategies for intracellular multiplication and nutrient acquisition. The book is recommended for all microbiology libraries.

Further details at Legionella: Molecular Microbiology

Microbes and the Law

A course on Microbes and the Law will take place 6-7 of October, 2008 in Uppsala, Sweden. It is aimed at PhD-students in the later part of their education and junior scientists. A symposium on the same topic will be held 8-9 of October with invited speakers from all over the world.

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The Cyanobacteria: Molecular Biology, Genomics and Evolution

The editors and authors of this book have succeeded admirably in producing a comprehensive, well-written book on the molecular biology, genomics and evolution of cyanobacteria. The breadth of topics covered is impressive, ranging from the crucial roles played by cyanobacteria in both the primitive and present Earth to the "nuts and bolts" of their cellular functioning. Advocates of whole genome sequences will not be disappointed, as a number of the chapters scrutinise the latest data to good effect. Whilst not strictly within the scope of the book, it is still somewhat disappointing that more information on cyanobacterial proteomics was not included to complement the genomic studies cited. However, this is a minor quibble. The greatest strength of this book is that whilst its in-depth treatment of various topics will satisfy the experienced cyanobacterial researcher, the easy to understand description of basics will also attract neophyte "cyanobacteriologists" and readers from other fields. This book deserves to become a standard reference on the molecular biology, genomics and evolution of cyanobacteria.

Review by: Dr. Roberto Anitori, Dept of Chemistry and Biomolecular Sciences, Macquarie University, North Ryde, NSW 2109, Australia

Full details of the book available at The Cyanobacteria: Molecular Biology, Genomics and Evolution