An 8 kb nucleotide sequence at the 3' flanking region of the sspC gene (184 degrees) on the Bacillus subtilis 168 chromosome containing an intein and an intron

Tying down the arm in Bacillus dUTPase: structure and mechanism

Homotrimeric dUTPases contain three active sites, each formed by five conserved sequence motifs originating from all three subunits. The essential fifth motif lies in a flexible C-terminal arm which becomes ordered during catalysis and is disordered in most crystal structures. Previously, it has been shown that the two Bacillus subtilis dUTPases, YncF and YosS, differ from their orthologues in the position in the sequence of the essential Phe-lid residue, which stacks against the uracil base, and in the conformation of the general base aspartate, which points away from the active site. Here, three structures of the complex of YncF with dU-PPi-Mg(2+) and the structure of YosS complexed with dUMP are reported. dU-PPi-Mg(2+) triggers the ordering of both the C-terminal arm and a loop (residues 18-26) which is uniquely disordered in the Bacillus dUTPases. The dUMP complex suggests two stages in substrate release. Limited proteolysis experiments allowed those complexes in which C-terminal cleavage is hindered and those in which it can be assumed to be ordered to be identified. The results lead to the suggestion that dUpNHpp is not a perfect substrate mimic, at least for the B. subtilis enzymes, and provide new insights into the mechanism of these two dUTPases in comparison to their orthologues. The enzyme mechanism is reviewed using the present and previous crystal structures as snapshots along the reaction coordinate.

Algal viruses with distinct intraspecies host specificities include identical intein elements

Heterosigma akashiwo virus (HaV) is a large double-stranded DNA virus infecting the single-cell bloom-forming raphidophyte (golden brown alga) H. akashiwo. A molecular phylogenetic sequence analysis of HaV DNA polymerase showed that it forms a sister group with Phycodnaviridae algal viruses. All 10 examined HaV strains, which had distinct intraspecies host specificities, included an intein (protein intron) in their DNA polymerase genes. The 232-amino-acid inteins differed from each other by no more than a single nucleotide change. All inteins were present at the same conserved position, coding for an active-site motif, which also includes inteins in mimivirus (a very large double-stranded DNA virus of amoebae) and in several archaeal DNA polymerase genes. The HaV intein is closely related to the mimivirus intein, and both are apparently monophyletic to the archaeal inteins. These observations suggest the occurrence of horizontal transfers of inteins between viruses of different families and between archaea and viruses and reveal that viruses might be reservoirs and intermediates in horizontal transmissions of inteins. The homing endonuclease domain of the HaV intein alleles is mostly deleted. The mechanism keeping their sequences basically identical in HaV strains specific for different hosts is yet unknown. One possibility is that rapid and local changes in the HaV genome change its host specificity. This is the first report of inteins found in viruses infecting eukaryotic algae.

Bacterial thymidylate synthase with intein, group II Intron, and distinctive ThyX motifs

The ThyX class of thymidylate synthases was previously characterized by a common ThyX motif, RHRX7S. We report bacterial ThyX sequences having distinctive ThyX motifs, suggesting a more general ThyX motif, R/THRX7-8S. One ThyX sequence has an intein in its ThyX motif that was shown to do protein splicing and a group II intron in its gene, suggesting a hot spot for these self-splicing mobile elements.

Evolution and horizontal transfer of dUTPase-encoding genes in viruses and their hosts

dUTPase is a ubiquitous and essential enzyme responsible for regulating cellular levels of dUTP. The dut gene exists as single, tandemly duplicated, and tandemly triplicated copies. Crystallized single-copy dUTPases have been shown to assemble as homotrimers. dUTPase is encoded as an auxiliary gene in a number of virus genomes. The origin of viral dut genes has remained unresolved since their initial discovery. A comprehensive analysis of dUTPase amino acid sequence relationships was performed to explore the evolutionary dynamics of dut in viruses and their hosts. Our data set, comprised of 24 host and 51 viral sequences, includes representative sequences from available eukaryotes, archaea, eubacteria cells, and viruses, including herpesviruses. These amino acid sequences were aligned by using a hidden Markov model approach developed to align divergent data. Known secondary structures from single-copy crystals were mapped onto the aligned duplicate and triplicate sequences. We show how duplicated dUTPases might fold into a monomer, and we hypothesize that triplicated dUTPases also assemble as monomers. Phylogenetic analysis revealed at least five viral dUTPase sequence lineages in well-supported monophyletic clusters with eukaryotic, eubacterial, and archaeal hosts. We have identified all five as strong examples of horizontal transfer as well as additional potential transfer of dut genes among eubacteria, between eubacteria and viruses, and between retroviruses. The evidence for horizontal transfers is particularly interesting since eukaryotic dut genes have introns, while DNA virus dut genes do not. This implies that an intermediary retroid agent facilitated the horizontal transfer process between host mRNA and DNA viruses.