Abundance of phages across samples
Coverage calculations were limited to the window encompassing the region defined by the spacer hits (proto-spacers) and phage-associated genes. This was in order to make sure only phage abundance was measured if it was integrated in a bacterial genome. Metagenomic reads from each sample were mapped to phage contigs using BLASTN, requiring at least 80% of the read to align against the contig with at least 85% identity. To declare a phage contig exists in a particular sample, the phage region of the contig had to be covered by at least one read per base pair and at least 70% of the phage-region bases had to be covered (if the phage region was <2 kb, it was extended by 2 kb upstream and downstream). See the Supplemental Text for discussion on the parameters chosen for the sharing analysis. Coverage is reported in reads per kilobase per millions of reads (RPKM). Visual data on phage abundance across samples are found at http://www.weizmann.ac.il/molgen/Sorek/microbiome_phages/.
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