title: Endosymbiont gene functions impaired and rescued by polymerase infidelity at poly(A) tracts.
abstract: Among host-dependent bacteria that have evolved by extreme reductive genome evolution, long-term bacterial endosymbionts of insects have the smallest (160-790 kb) and most A + T-rich (>70%) bacterial genomes known to date. These genomes are riddled with poly(A) tracts, and 5-50% of genes contain tracts of 10 As or more. Here, we demonstrate transcriptional slippage at poly(A) tracts within genes of Buchnera aphidicola associated with aphids and Blochmannia pennsylvanicus associated with ants. Several tracts contain single frameshift deletions; these apparent pseudogenes showed patterns of constraint consistent with purifying selection on the encoded proteins. Transcriptional slippage yielded a heterogeneous population of transcripts with variable numbers of As in the tract. Across several frameshifted genes, including B. aphidicola cell wall biosynthesis genes and a B. pennsylvanicus histidine biosynthesis gene, 12-50% of transcripts contained corrected reading frames that could potentially yield full-length proteins. In situ immunostaining confirmed the production of the cell wall biosynthetic enzyme UDP-N-acetylmuramyl pentapeptide synthase encoded by the frameshifted murF gene. Simulation studies indicated an overrepresentation of poly(A) tracts in endosymbiont genomes relative to other A + T-rich bacterial genomes. Polymerase infidelity at poly(A) tracts rescues the functionality of genes with frameshift mutations and, conversely, reduces the efficiency of expression for in-frame genes carrying poly(A) regions. These features of homopolymeric tracts could be exploited to manipulate gene expression in small synthetic genomes.
authors.affiliation: Department of Molecular Evolution, Evolutionary Biology Centre, Uppsala University, SE-752 36 Uppsala, Sweden.
authors.forename: Ivica
authors.initials: I
authors.lastname: Tamas
authors.forename: Jennifer J
authors.initials: JJ
authors.lastname: Wernegreen
authors.forename: Björn
authors.initials: B
authors.lastname: Nystedt
authors.forename: Seth N
authors.initials: SN
authors.lastname: Kauppinen
authors.forename: Alistair C
authors.initials: AC
authors.lastname: Darby
authors.forename: Laura
authors.initials: L
authors.lastname: Gomez-Valero
authors.forename: Daniel
authors.initials: D
authors.lastname: Lundin
authors.forename: Anthony M
authors.initials: AM
authors.lastname: Poole
authors.forename: Siv G E
authors.initials: SG
authors.lastname: Andersson
mesh_terms: D000818:Animals; D001042:Aphids / Q000196:embryology / Q000201:enzymology / Q000382:microbiology; D001483:Base Sequence; D020635:Buchnera / Q000235:genetics* / Q000502:physiology; D002473:Cell Wall / Q000235:genetics; D003198:Computer Simulation; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism*; D004625:Embryo, Nonmammalian / Q000196:embryology / Q000201:enzymology / Q000382:microbiology; D019143:Evolution, Molecular; D016368:Frameshift Mutation; D015870:Gene Expression; D016680:Genome, Bacterial; D006639:Histidine / Q000096:biosynthesis; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D010453:Peptide Synthases / Q000096:biosynthesis / Q000235:genetics; D026723:Polyadenylation / Q000235:genetics*; D012333:RNA, Messenger / Q000096:biosynthesis* / Q000235:genetics; D013559:Symbiosis / Q000235:genetics*; D014158:Transcription, Genetic
chemical_list: D012333:RNA, Messenger; D006639:Histidine; D012321:DNA-Directed RNA Polymerases; D010453:Peptide Synthases; C060628:UDP-N-acetylmuramoylalanyl-D-glutamyllysine-D-alanyl-D-alanine ligase