Sergei A. Spirin

Solitary restriction endonucleases in prokaryotic genomes

Prokaryotic restriction-modification (R-M) systems defend the host cell from the invasion of a foreign DNA. They comprise two enzymatic activities: specific DNA cleavage activity and DNA methylation activity preventing cleavage. Typically, these activities are provided by two separate enzymes: a DNA methyltransferase (MTase) and a restriction endonuclease (RE). In the absence of a corresponding MTase, an RE of Type II R-M system is highly toxic for the cell. Genes of the R-M system are linked in the genome in the vast majority of annotated cases. There are only a few reported cases in which the genes of MTase and RE from one R-M system are not linked. Nevertheless, a few hundreds solitary RE genes are present in the Restriction Enzyme Database (http://rebase.neb.com) annotations. Using the comparative genomic approach, we analysed 272 solitary RE genes. For 57 solitary RE genes we predicted corresponding MTase genes located distantly in a genome. Of the 272 solitary RE genes, 99 are likely to be fragments of RE genes. Various explanations for the existence of the remaining 116 solitary RE genes are also discussed.

Structural Aspects of Interaction of Homeodomains with DNA

This review is devoted to the structural aspects of interaction of homeodomains with DNA. Presented are the list of all homeodomains with known spatial structure and the alignment of their amino acid sequences. The structure of homeodomains and contacts of their amino acid residues with DNA bases and sugar-phosphate backbone are described. The role of water molecules in DNA binding is discussed. Structures of multicomponent protein complexes on DNA including homeodomains are characterized.

New words in human mutagenesis

BackgroundThe substitution rates within different nucleotide contexts are subject to varying levels of bias. The most well known example of such bias is the excess of C to T (C > T) mutations in CpG (CG) dinucleotides. The molecular mechanisms underlying this bias are important factors in human genome evolution and cancer development. The discovery of other nucleotide contexts that have profound effects on substitution rates can improve our understanding of how mutations are acquired, and why mutation hotspots exist.ResultsWe compared rates of inherited mutations in 1-4 bp nucleotide contexts using reconstructed ancestral states of human single nucleotide polymorphisms (SNPs) from intergenic regions. Chimp and orangutan genomic sequences were used as outgroups. We uncovered 3.5 and 3.3-fold excesses of T > C mutations in the second position of ATTG and ATAG words, respectively, and a 3.4-fold excess of A > C mutations in the first position of the ACAA word.ConclusionsAlthough all the observed biases are less pronounced than the 5.1-fold excess of C > T mutations in CG dinucleotides, the three 4 bp mutation contexts mentioned above (and their complementary contexts) are well distinguished from all other mutation contexts. This provides a challenge to discover the underlying mechanisms responsible for the observed excesses of mutations.

Effector proteins of chlamydiae

This review summarizes the recently published data on the molecular mechanisms of Chlamydiae-host cell interaction, first of all, on chlamydial effector proteins. Such proteins, along with type III transport system proteins, which transfer many effector proteins into the host cytoplasm, are attractive targets for drug therapy of chlamydial infections. The majority of the data concerns two species, Chlamydia trachomatis and Chlamydophila pneumoniae. The C. trachomatis protein TARP, which is presynthesized in elementary bodies, plays an essential role in the initial stages of infection. The pathogen proteins that are involved in the next stage, which is the intracellular inclusion traffic to the centrosome, are C. trachomatis CT229 and C. pneumoniae Cpn0585, which interact with cell Rab GTPases. In C. trachomatis, IncA plays a key role in the fusion of chlamydial inclusions, CT847 modulates the life cycle of the host cell, and LDA3 is essential for the acquisition of nutrients. The protease CPAF and the inclusion membrane proteins IncG and CADD are involved in suppressing apoptosis of infected cells. The proteases CPAF and CT441 and the deubiquitinating protein ChlaDub1 help the pathogen to evade the immune response.

Role of restriction-modification systems in prokaryotic evolution and ecology

Restriction–modification (R-M) systems are able to methylate or cleave DNA depending on methylation status of their recognition site. It allows them to protect bacterial cells from invasion by foreign DNA. Comparative analysis of a large number of available bacterial genomes and methylomes clearly demonstrates that the role of R-M systems in bacteria is wider than only defense. R-M systems maintain heterogeneity of a bacterial population and are involved in adaptation of bacteria to change in their environmental conditions. R-M systems can be essential for host colonization by pathogenic bacteria. Phase variation and intragenomic recombinations are sources of the fast evolution of the specificity of R-M systems. This review focuses on the influence of R-M systems on evolution and ecology of prokaryotes.

IDENTIFICATION OF CONSERVED FEATURES OF LAGLIDADG HOMING ENDONUCLEASES

LAGLIDADG family of homing endonucleases are rare-cutting enzymes which recognize long target sequences and are of great interest in genome engineering. Despite advances in homing endonuclease engineering, effective methods of broadening the range of cleaved sequences are still lacking. Here, we present a study of conserved structural features of LAGLIDADG homing endonucleases that might aid further development of such methods. The protein-DNA interface of LAGLIDADG homing endonucleases differs considerably with the particular nuclease, and the analysis of conserved protein-DNA interactions could not identify any residues crucial for DNA binding and common to most nucleases of the family. For the homing endonuclease PI-SceI, a comparison of structural and experimental data derived from literature helped to identify 23 residues that are likely to be important for DNA binding. Analysis of the LAGLIDADG domain dimerization interface allowed the choosing of six positions that contribute to dimerization specificity most, while comparison of 446 sequences of LAGLIDADG endonucleases revealed groups of residues in these positions that appear to be most favorable for dimerization.

An updated version of NPIDB includes new classifications of DNA–protein complexes and their families

The recent upgrade of nucleic acid–protein interaction database (NPIDB, http://npidb.belozersky.msu.ru/) includes a newly elaborated classification of complexes of protein domains with double-stranded DNA and a classification of families of related complexes. Our classifications are based on contacting structural elements of both DNA: the major groove, the minor groove and the backbone; and protein: helices, beta-strands and unstructured segments. We took into account both hydrogen bonds and hydrophobic interaction. The analyzed material contains 1942 structures of protein domains from 748 PDB entries. We have identified 97 interaction modes of individual protein domain–DNA complexes and 17 DNA–protein interaction classes of protein domain families. We analyzed the sources of diversity of DNA–protein interaction modes in different complexes of one protein domain family. The observed interaction mode is sometimes influenced by artifacts of crystallization or diversity in secondary structure assignment. The interaction classes of domain families are more stable and thus possess more biological sense than a classification of single complexes. Integration of the classification into NPIDB allows the user to browse the database according to the interacting structural elements of DNA and protein molecules. For each family, we present average DNA shape parameters in contact zones with domains of the family.

[Conserved structural features of ETS domain--DNA complexes].

ETS proteins are a family of widespread transcription factors that regulate the expression of many animal genes. Structurally, ETS proteins are characterized by a conserved DNA-binding ETS domain, which recognizes DNA sequences containing the trinucleotide GGA. The structural features of ETS domain-DNA complexes were analyzed, and conserved contacts important in terms of interaction stability and specificity were identified. The analysis revealed nine conserved hydrogen bonds with oxygens of DNA backbone phosphates, two bidentate hydrogen bonds with DNA major groove atoms, one conserved hydrophobic cluster located on the protein-DNA interface and important for binding site recognition, and 12 conserved water molecules presumably mediating the ETS domain-DNA interaction. The results are represented in specialized data bank of protein-DNA complexes (NPIDB).

The Role of Water in Homeodomain-DNA Interaction

Interfacial water molecules in homeodomain-DNA complexes were analyzed involving all available 3D structures of homeodomains. The main results are as follows: (i) eight conserved water bridges between homeodomains and DNA phosphate groups were identified, as compared to ten ‘direct’ conserved hydrogen bonds with phosphates; (ii) conserved water molecules mediating contacts with DNA phosphate groups seem to be preorganized on an isolated homeodomain surface; (iii) water-mediated contacts of homeodomains with the DNA major groove could contribute to homeodomain specificity in addition to the direct hydrogen bonds and hydrophobic contacts; and (iv) almost all places for potential water bridges on the homeodomain-DNA interface are occupied by water molecules identified by X-ray crystallography.

ANALYSIS OF CONSERVED HYDROPHOBIC CORES IN PROTEINS AND SUPRAMOLECULAR COMPLEXES

The conserved hydrophobic core is an important feature of a family of protein domains. We suggest a procedure for finding and the analysis of conserved hydrophobic cores. The procedure is based on using an original program called CluD (http://monkey.belozersky.msu.ru/CluD/cgi-bin/hftri.pl). Conserved hydrophobic cores of several families including homeodomains and interlock-containing domains are described. Hydrophobic clusters on some protein-DNA and protein-protein interfaces were also analyzed.