Three cos-type virulent Streptococcus thermophilus phages were isolated from failed mozzarella production in Uruguay. Genome analyses showed that these phages are similar to those isolated elsewhere around the world. The CRISPR1 and CRISPR3 arrays of the three S. thermophilus host strains from Uruguay were also characterized and similarities were noted with previously described model strains SMQ-301, LMD-9 and DGCC7710. Spontaneous bacteriophage-insensitive S. thermophilus mutants (BIMs) were obtained after challenging the phage-sensitive wild-type strain Uy02 with the phage 128 and their CRISPR content was analyzed. Analysis of 23 BIMs indicated that all of them had acquired at least one new spacer in their CRISPR1 array. While 14 BIMs had acquired spacer at the 5′-end of the array, 9 other BIMs acquired a spacer within the array. Comparison of the leader sequence in strains Uy02 and DGCC7710 showed a nucleotide deletion at position -1 in Uy02, which may be responsible for the observed ectopic spacer acquisition. Analysis of the spacer sequences upstream the newly acquired ectopic spacer indicated presence of a conserved adenine residue at position -2. This study indicates that natural strains of S. thermophilus can also acquire spacers within a CRISPR array.
CRISPR-Cas systems in prokaryotic cells provide an adaptive immunity against invading nucleic acids. For example, phage infection leads to addition of new immunity (spacer acquisition) and DNA cleavage (interference) in the bacterial model species Streptococcus thermophilus, which primarily relies on Cas9-containing CRISPR-Cas systems. Phages can counteract this defense system through mutations in the targeted protospacers or by encoding anti-CRISPR proteins (ACRs) that block Cas9 interference activity. Here, we show that S. thermophilus can block ACR-containing phages when the CRISPR immunity specifically targets the acr gene. This in turn selects for phage mutants carrying a deletion within the acr gene. Remarkably, a truncated acrIIA allele, found in a wild-type virulent streptococcal phage, does not block the interference activity of Cas9 but still prevents the acquisition of new immunities, thereby providing an example of an ACR specifically inhibiting spacer acquisition.
Streptococcus thermophilus relies heavily on two type II-A CRISPR-Cas systems, CRISPR1 and CRISPR3, to resist siphophage infections. One hallmark of these systems is the integration of a new spacer at the 5′ end of the CRISPR arrays following phage infection. However, we have previously shown that ectopic acquisition of spacers can occur within the CRISPR1 array. Here, we present evidence of the acquisition of new spacers within the array of CRISPR3 of S. thermophilus. The analysis of randomly selected bacteriophage-insensitive mutants of the strain Uy01 obtained after phage infection, as well as the comparison with other S. thermophilus strains with similar CRISPR3 content, showed that a specific spacer within the array could be responsible for misguiding the adaptation complex. These results also indicate that while the vast majority of new spacers are added at the 5′ end of the CRISPR array, ectopic spacer acquisition is a common feature of both CRISPR1 and CRISPR3 systems in S. thermophilus, and it can still provide phage resistance. Ectopic spacer acquisition also appears to have occurred naturally in some strains of Streptococcus pyogenes, suggesting that it is a general phenomenon, at least in type II-A systems.
From October to December 2005, onion (Allium cepa) plants in seed-production fields in south Uruguay (Canelones) had symptoms suggestive of those caused by Iris yellow spot virus (IYSV; genus Tospovirus, family Bunyaviridae). Symptoms included diamond-shaped lesions on seed stalks (scapes), each 1 to 5 cm long with a necrotic border, green center, and sometimes a second necrotic area in the center of the diamond (2,3). Necrotic lesions with more irregular shape were also associated with diseased plants. In 2006, scape samples with these symptoms were collected from four onion seed crops and assayed for IYSV using an IYSV-specific antiserum (Agdia Inc., Elkhart, IN) in a double-antibody sandwich-ELISA. IYSV was detected in all four onion seed crops monitored in 2006. IYSV incidence, expressed as the number of plants with symptoms, ranged from <1% (1 of 120 plants evaluated) to 20% (24 of 120 plants). Two fields were monitored in 2007, in which IYSV incidence increased from 2 and 3% in October to 7% (198 of 2,768 plants) and 40% (253 of 638 plants) in December, respectively. The highest incidence was observed in the same farm in 2006 and 2007. Scapes were sampled from the field with the highest incidence of symptoms in 2007 and tested for IYSV with IYSV-specific primers (3). Total RNA was extracted from 100 mg of symptomatic tissue, with green tissue adjacent to typical lesions, following a Trizol-based protocol (1). A reverse transcriptase-PCR assay with nucleocapsid gene-specific primers was used (3). A PCR product of approximately 26 bp was obtained, coincident with the expected length for IYSV. The PCR product was cloned and sequenced. The tospovirus N sequence of the isolate in Uruguay (Accession No. GU550518) had maximum identity (97%) with an Australian IYSV isolate (Accession No. AY345227), and >87% identity only with IYSV N protein sequences in GenBank. Because of the presence of IYSV in Brazil, Chile, and Peru, this first documentation, to our knowledge, of IYSV in onion crops in Uruguay suggests that the threat of IYSV to onion is increasing in South America. References: (1) P. Chomczynski and K. Mackey. Biotechniques 19:942, 1995. (2) D. H. Gent et al. Plant Dis. 90:1468, 2006. (3) H. R. Pappu et al. Plant Dis. 92:588, 2008.
It contained an error in Fig. 4A, in which a 6-amino acid insertion (positions 101-106) was incorrectly shown in protein AcrIIA6 123, compared to AcrIIA6 D1811. The correct figure now shows that this 6-amino acid sequence is also present in AcrIIA6 D1811.It contained an error in Fig. 4C, in which a 6-amino acid insertion (positions 101-106) was incorrectly highlighted using green colour. This highlighting has been removed from the figure.It contained an error in Fig. 4E, in which a 6-amino acid insertion (positions 101-106) was incorrectly highlighted using a dotted ellipse. The dotted ellipse has been removed from the figure.It contained errors in the legend of Fig. 4, which incorrectly read 'Red and green represent amino acid substitutions and amino acid insertions, respectively' and 'The dotted ellipse highlights the position of the amino acid insertion in AcrIIA6 123 '. The correct version replaces the first sentence with 'Red patches represent amino acid substitutions', and removes the second sentence.It contained an error in a sentence of the 'Results and discussion' section, which incorrectly read 'This 3D model of AcrIIA6 123 highlights a 6-residue insertion (K101-I106) in the loop connecting the β-sheet to the C-terminal α-helix, as well as some amino acid substitutions distributed over the entire structure'. The correct version replaces this sentence with 'This 3D model of AcrIIA6 123 highlights amino acid substitutions distributed over the entire structure'.It contained an error in a sentence of the 'Results and discussion' section, which incorrectly read 'Interestingly, the monomeric AcrIIA6 123 presents three notable features by (1) offering a much smaller binding surface than that of the dimeric AcrIIA6, (2) harboring the amino acid insertion in an RNA-interacting loop, and (3) containing an amino acid substitution (I23 in AcrIIA6 123 instead of N81 in AcrIIA6) that likely disrupts the St1Cas9-RNA-binding interface'. The correct version replaces this sentence with 'Interestingly, the monomeric AcrIIA6 123 presents two notable features by (1) offering a much smaller binding surface than that of the dimeric AcrIIA6, and (2) containing an amino acid substitution (I23 in AcrIIA6 123 instead of N81 in AcrIIA6) that likely disrupts the St1Cas9-RNA-binding interface'.The errors have been corrected in both the PDF and HTML versions of the Article.
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