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Staphylococcal SCCmec elements encode an active MCM-like helicase and thus may be replicative

Nature Structural & Molecular Biology volume 23, pages 891898 (2016) | Download Citation

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is a public-health threat worldwide. Although the mobile genomic island responsible for this phenotype, staphylococcal cassette chromosome (SCC), has been thought to be nonreplicative, we predicted DNA-replication-related functions for some of the conserved proteins encoded by SCC. We show that one of these, Cch, is homologous to the self-loading initiator helicases of an unrelated family of genomic islands, that it is an active 3′-to-5′ helicase and that the adjacent ORF encodes a single-stranded DNA–binding protein. Our 2.9-Å crystal structure of intact Cch shows that it forms a hexameric ring. Cch, like the archaeal and eukaryotic MCM-family replicative helicases, belongs to the pre–sensor II insert clade of AAA+ ATPases. Additionally, we found that SCC elements are part of a broader family of mobile elements, all of which encode a replication initiator upstream of their recombinases. Replication after excision would enhance the efficiency of horizontal gene transfer.

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Acknowledgements

We thank R. Daum (Department of Pediatrics and MRSA Center, University of Chicago) for providing USA300 MRSA strain 923, from which we cloned Cch and LP1413, J.R. Penadés and M.R. Boocock for insightful discussions, J. Herrou for assistance with crystallization, Y.-L. Chan for biochemical advice, the CCP4 summer school for crystallographic advice and the staff of the Structural Biology Center at Argonne National Laboratory for assistance with data collection. This work was funded by grant R21AI117593 (NIAID, NIH ) to P.A.R.

Author information

Author notes

    • Agnieszka Misiura

    Present address: Abbott Laboratories, North Chicago, Illinois, USA.

Affiliations

  1. Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, Illinois, USA.

    • Ignacio Mir-Sanchis
    • , Christina A Roman
    • , Agnieszka Misiura
    • , Ying Z Pigli
    •  & Phoebe A Rice
  2. Department of Pediatrics, The University of Chicago, Chicago, Illinois, USA.

    • Susan Boyle-Vavra
  3. MRSA Research Center, The University of Chicago, Chicago, Illinois, USA.

    • Susan Boyle-Vavra

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Contributions

P.A.R. conceived and directed the project and carried out most of the bioinformatics. I.M.-S. carried out all of the crystallography and biochemistry shown in the figures as well as some of the bioinformatics. S.B.-V. advised on staphylococcal molecular biology, suggested examining the cch gene and carried out the initial cloning of cch. A.M. contributed to the cloning and initial stages of the Cch biochemistry; C.A.R. worked out the purification and initial characterization of LP1413; and Y.Z.P. assisted with protein purification and crystallization. I.M.-S. and P.A.R. wrote the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Phoebe A Rice.

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DOI

https://doi.org/10.1038/nsmb.3286

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