Page 2 - Bacteriophage genes that inactivate the CRISPR/Cas bacterial immune system

Bondy-Denomy et al. Page 2

thus engendering resistance to phages and other invading DNA molecules 10,11 . Cas proteins
also mediate an adaptive function by incorporating short sequences (i.e. a new spacer) from
newly encountered foreign genomes into CRISPR loci so that these genomes will be
destroyed in subsequent encounters 12–14 . The relatively recent elucidation of CRISPR/Cas
functions and their obvious similarities to RNAi systems of eukaryotes have led to vigorous
investigation of these systems.

Since phage genes have been discovered that can neutralize most of the prevalent bacterial
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anti-phage defences , the failure to identify genes that counteract the widely occurring
CRISPR/Cas systems was surprising. To search for such “anti-CRISPR” activity, we
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investigated the Type I-F CRISPR/Cas system of the opportunistic pathogen Pseudomonas
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aeruginosa utilizing a collection of 44 lysogens of P. aeruginosa PA14, which each
contained a different phage genome (see Methods). In lysogens, phage genomes are
integrated into the bacterial genome and are referred to as prophages. Although prophage
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genes are generally repressed, all prophages have some genes that are actively transcribed.
To test whether prophages might express anti-CRISPR activity, we measured the plaquing
efficiency of three “CRISPR-sensitive” phages (JBD18, JBD25, and JBD67) on our
collection of lysogens. The CRISPR-sensitive phages fail to replicate on PA14 due to the
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action of the CRISPR/Cas system , but are able to replicate on PA14 ΔCR/cas, which
contains no CRISPR loci or cas genes (Fig. 1a, Supplementary Fig. 1a). We identified three
lysogenic strains, PA14(JBD24), PA14(MP29), and PA14(JBD30), on which the CRISPR-
sensitive phages could form plaques very robustly as compared to unlysogenized PA14 (Fig.
1a; Supplementary Fig. 1a). Notably, the plaquing efficiency of the CRISPR-sensitive
phages on PA14(JBD30) was equivalent to that on the ΔCR/cas strain, indicating that the
JBD30 prophage caused complete inactivation of the CRISPR/Cas system. The somewhat
lower plaquing efficiency of the CRISPR-sensitive phages on the other lysogens relative to
their plaquing on the ΔCR/cas strain may be due to their production of less potent anti-
CRISPR activity. However, these prophages also attenuate plaquing through mechanisms
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independent of the CRISPR/Cas system as is demonstrated by the partial inhibition of
plaquing of the control phage, DMS3, which is not affected by the CRISPR/Cas system (Fig.
1a and Supplementary Fig. 1a).

To directly assess the anti-CRISPR activity of the PA14 lysogens, we used a plasmid-based
transformation efficiency assay. The sequences within phages that are targeted by the
CRISPR/Cas system are called protospacers. In order to be targeted, a protospacer sequence
must be complementary to a specific spacer sequence within the CRISPR locus and also
possess a correct Protospacer Adjacent Motif (PAM) 15,16 (Fig. 1b). Protospacer sequences
are named according to the spacer sequence that they match in the PA14 genome (Fig. 1b).
We constructed plasmids containing targeted protospacer sequences from phages JBD18
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(CRISPR2 locus, spacer 1 or CR2_sp1) and JBD25 (CR1_sp1) . The transformation
efficiencies of the plasmids bearing protospacers into unlysogenized PA14 were reduced by
at least 90% compared to an empty vector control, whereas no difference in transformation
efficiency was seen for the three strains containing prophages expressing anti-CRISPR
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activity, or for the ΔCR/cas strain (Fig. 1c). These data confirm that the prophages isolated
in our screen inhibit the PA14 CRISPR/Cas system.

Nature. Author manuscript; available in PMC 2016 July 04.