Exploring Genomic Conservation in Actinobacteriophages with Small Genomes

Abstract

Actinobacteriophages of a wide range of genome sizes continue to be isolated and characterized, but only a handful of these have atypically small genomes, defined in this work as genome sizes under 20,000 bp. These small phages are relatively rare and have received minimal study thus far. Small phages have been isolated on Arthrobacter, Gordonia, Rhodococcus, and Microbacterium hosts. A previous study by Pope et al. showed that Gordonia small phages have similar gene products and amino acid sequences. Here, we set out to further examine relationships between small Gordonia phages as well as small phages that infect other hosts. Of the 3222 sequenced phages listed on PhagesDB, we identified 109 distinctly small phages with genome sizes under 20,000 bp. The majority of the small phages were isolated on Arthrobacter or Microbacterium hosts. Using comparative genomics, we searched for patterns of similarity among 34 cluster-representative small phages. Dot plot comparisons showed that there was more amino acid conservation than nucleotide identity amongst small phages. Gene content similarity (GCS) analysis revealed that the temperate Gordonia phages in Cluster CW share significant GCS values (over 35%) with the lytic Arthrobacter phages in Cluster AN, suggesting that some small phages have a considerable degree of genomic similarity with each other. SplitsTree analyses of shared phams (genes with substantial amino acid identity) supported the complexity of clustering criteria in small phages, given shuffling of genes across phages of different clusters and close relationships despite varied cluster membership. We observed this continuum of phage diversity through Rhodococcus the closer similarity of phage RRH1 to phages in Gordonia subcluster CW1 than CW1 is to Gordonia subcluster CW3. Finally, we were able to confirm the presence of conserved phams across not only small Gordonia phages but also within small phages from different clusters and hosts. Studying these genomic trends hidden in small phages allows us to better understand and appreciate the overall diversity of phages.

Prophage-Derived Regions in Curtobacterium Genomes: Good Things, Small Packages

“…However, a small likeness of the order of 10% has been detected with some phages infecting Microbacterium bacteria, which is phylogenetically close to the genus Curtobacterium . In particular, small Microbacterium phages with a genome size under 20,000 bp [ 57 ], recently assigned to newly established taxa (family Orlajensenviridae , subfamily Pelczarvirinae , genus Paopuvirus ) [ 58 ], were shown to share distant intergenomic similarities with putative Curtobacterium prophages, with a similar genome size, assigned to Group 13.…”

Characterization of Genomic Diversity In Bacteriophages InfectingRhodococcus

“…RRH1 is the only phage in this analysis to have been isolated on the host type of Rhodococcus rhodochrous Rrho39 (Supplemental Table 1). This singleton has the smallest genome length out of all the sequenced Rhodococcus phages – just 14,270 bp – which is even small relative to the general actinobacteriophage population (Tse et al, 2022). In addition to having the smallest genome length, RRH1 has a G+C% content of 68.4%, which is the highest G+C% content value out of all the other sequenced Rhodococcus phages (Supplemental Table 1, Figure 5).…”