Single-strand DNA processing: phylogenomics and sequence diversity of a superfamily of potential prokaryotic HuH endonucleases

Abstract

BackgroundSome mobile genetic elements target the lagging strand template during DNA replication. Bacterial examples are insertion sequences IS608 and ISDra2 (IS200/IS605 family members). They use obligatory single-stranded circular DNA intermediates for excision and insertion and encode a transposase, TnpAIS200, which recognizes subterminal secondary structures at the insertion sequence ends. Similar secondary structures, Repeated Extragenic Palindromes (REP), are present in many bacterial genomes. TnpAIS200-related proteins, TnpAREP, have been identified and could be responsible for REP sequence proliferation. These proteins share a conserved HuH/Tyrosine core domain responsible for catalysis and are involved in processes of ssDNA cleavage and ligation. Our goal is to characterize the diversity of these proteins collectively referred as the TnpAY1 family.ResultsA genome-wide analysis of sequences similar to TnpAIS200 and TnpAREP in prokaryotes revealed a large number of family members with a wide taxonomic distribution. These can be arranged into three distinct classes and 12 subclasses based on sequence similarity. One subclass includes sequences similar to TnpAIS200. Proteins from other subclasses are not associated with typical insertion sequence features. These are characterized by specific additional domains possibly involved in protein/DNA or protein/protein interactions. Their genes are found in more than 25% of species analyzed. They exhibit a patchy taxonomic distribution consistent with dissemination by horizontal gene transfers followed by loss. The tnpAREP genes of five subclasses are flanked by typical REP sequences in a REPtron-like arrangement. Four distinct REP types were characterized with a subclass specific distribution. Other subclasses are not associated with REP sequences but have a large conserved domain located in C-terminal end of their sequence. This unexpected diversity suggests that, while most likely involved in processing single-strand DNA, proteins from different subfamilies may play a number of different roles.ConclusionsWe established a detailed classification of TnpAY1 proteins, consolidated by the analysis of the conserved core domains and the characterization of additional domains. The data obtained illustrate the unexpected diversity of the TnpAY1 family and provide a strong framework for future evolutionary and functional studies. By their potential function in ssDNA edition, they may confer adaptive responses to host cell physiology and metabolism.

Association of lanthipeptide genes with TnpAREPtransposases in marine picocyanobacteria

“…Gene annotation, sequence alignments, and phylogenetic tree construction. Prochlorosin annotations were taken from [1] , and TnpA REP from [18], [24] . Prediction of procA and tnpA REP genes was expanded to new strains using BLASTP (E value of 1e-5) with ProcA or TnpA REP protein sequence query.…”

“…We found a number of TnpA REP family transposase homologs in marine picocyanobacteria, all of them forming a monophyletic group that appears to be descended from TnpA REP in other cyanobacteria such as Trichodesmium erythraeum (Sup Fig 1, 2). They belong to a subclass of TnpA REP (RAYT group 1 in [18] , or subclass 2.1 in [24] ) which possess an additional conserved domain in the C-terminal region, likely involved in DNA-binding [24] . In marine picocyanobacteria, the tnpA REP genes tend to co-localize with clusters or individual procA genes, which encode for the peptides that are cyclized by ProcM to become the final product: prochlorosins ( Fig.…”

“…To begin to unravel the mechanism of association we searched for potential TnpA REP recognition targets. Of note, marine picocyanobacteria belong to a TnpA REP subgroup that was not found to associate with REP-like sequences, but rather with inverted repeats (consensus sequence GGGG[AT][CG]A[CG]) [24], [25] . We could not find any sign of association with inverted repeats, but we did detect motifs forming DNA secondary structures: a hairpin structure (Fig 2A) in the intergenic sequence, and a Guanine quadruplex (G4) structure (Fig 2A) (see methods for details); both of these structure types have been shown to be potential recognition targets for TnpA REP transposases [16], [17], [23] .…”