Archaeal Hel308 suppresses recombination through a catalytic switch that controls DNA annealing.

Hel308 helicases promote genome stability in archaea and are conserved in metazoans, where they are known as HELQ. Their helicase mechanism is well characterised, but it is unclear how they specifically contribute to genome stability in archaea. We show here that a highly conserved motif of Hel308/HELQ helicases (motif IVa, F/YHHAGL) modulates both DNA unwinding and a newly identified strand annealing function of archaeal Hel308. A single amino acid substitution in motif IVa results in hyper-active DNA helicase and annealase activities of purified Hel308 in vitro. All-atom molecular dynamics simulations using Hel308 crystal structures provided a molecular basis for these differences between mutant and wild type Hel308. In archaeal cells, the same mutation results in 160000-fold increased recombination, exclusively as gene conversion (non-crossover) events. However, crossover recombination is unaffected by the motif IVa mutation, as is cell viability or DNA damage sensitivity. By contrast, cells lacking Hel308 show impaired growth, increased sensitivity to DNA cross-linking agents, and only moderately increased recombination. Our data reveal that archaeal Hel308 suppresses recombination and promotes DNA repair, and that motif IVa in the RecA2 domain acts as a catalytic switch to modulate the separable recombination and repair activities of Hel308.

Escherichia coli DNA repair helicase Lhr is also a uracil-DNA glycosylase.

DNA glycosylases protect genetic fidelity during DNA replication by removing potentially mutagenic chemically damaged DNA bases. Bacterial Lhr proteins are well-characterized DNA repair helicases that are fused to additional 600-700 amino acids of unknown function, but with structural homology to SecB chaperones and AlkZ DNA glycosylases. Here, we identify that Escherichia coli Lhr is a uracil-DNA glycosylase (UDG) that depends on an active site aspartic acid residue. We show that the Lhr DNA helicase activity is functionally independent of the UDG activity, but that the helicase domains are required for fully active UDG activity. Consistent with UDG activity, deletion of lhr from the E. coli chromosome sensitized cells to oxidative stress that triggers cytosine deamination to uracil. The ability of Lhr to translocate single-stranded DNA and remove uracil bases suggests a surveillance role to seek and remove potentially mutagenic base changes during replication stress.

Mechanistic insights into Lhr helicase function in DNA repair.

The DNA helicase Large helicase-related (Lhr) is present throughout archaea, including in the Asgard and Nanoarchaea, and has homologues in bacteria and eukaryotes. It is thought to function in DNA repair but in a context that is not known. Our data show that archaeal Lhr preferentially targets DNA replication fork structures. In a genetic assay, expression of archaeal Lhr gave a phenotype identical to the replication-coupled DNA repair enzymes Hel308 and RecQ. Purified archaeal Lhr preferentially unwound model forked DNA substrates compared with DNA duplexes, flaps and Holliday junctions, and unwound them with directionality. Single-molecule FRET measurements showed that binding of Lhr to a DNA fork causes ATP-independent distortion and base-pair melting at, or close to, the fork branchpoint. ATP-dependent directional translocation of Lhr resulted in fork DNA unwinding through the 'parental' DNA strands. Interaction of Lhr with replication forks in vivo and in vitro suggests that it contributes to DNA repair at stalled or broken DNA replication.

A systematic review of early postpartum medroxyprogesterone receipt and early breastfeeding cessation: evaluating the methodological rigor of the evidence.

BACKGROUND: Breastfeeding has numerous maternal and infant benefits. Progesterone contraception after birth is frequently recommended, but because a decrease in progesterone is required to initiate lactation, early postpartum progesterone contraception use could inhibit lactation. The purpose of this article is to critically evaluate the scientific basis for conflicting clinical recommendations related to postpartum medroxyprogesterone use among breastfeeding women. METHODS: Relevant peer-reviewed literature was identified through a comprehensive search of PubMed through December 2010. The search was restricted to clinical trials, randomized clinical trials, or comparative studies written in English and conducted among humans. The studies included in this review addressed the effect of medroxyprogesterone administration at <6 weeks postpartum on breastfeeding exclusivity and/or duration and measured breastfeeding outcomes at ≥ 6 weeks postpartum. RESULTS: Of the 20 articles identified, only three studies satisfied the inclusion criteria. However, all three studies were of low-quality methodological rigor, and none accounted for potential confounders. CONCLUSION: Current evidence is methodologically weak and provides an inadequate basis for inference about a possible causal relationship between early postpartum medroxyprogesterone use and poor breastfeeding outcomes. However, given the presence of a strong biological model describing the potential deleterious effect of postpartum medroxyprogesterone use on lactation, further research that improves on current literature is warranted. Meanwhile, we recommend that potential breastfeeding risks associated with early (<6 weeks) postpartum medroxyprogesterone use be disclosed to allow for a fully informed consent and decision-making process.