Birth of a retroposon: the Twin SINE family from the vector mosquito Culex pipiens may have originated from a dimeric tRNA precursor.
SINEs are short interspersed repetitive elements found in many eukaryotic genomes and are believed to propagate by retroposition. Almost all SINEs reported to date have a composite structure made of a 5' tRNA-related region followed by a tRNA-unrelated region. Here, we describe a new type of tRNA-derived SINEs from the genome of the mosquito Culex pipiens. These elements, called TWINs, are approximately 220 bp long and reiterated at approximately 500 copies per haploid genome. TWINs have a unique structure compared with other tRNA-SINEs described so far. They consist of two tRNA(Arg)-related regions separated by a 39-bp spacer. Other tRNA-unrelated sequences include a 5-bp leader preceding the left tRNA-like unit and a short trailer located downstream of the right tRNA-like region. This 3' trailer is a 10-bp sequence that is ended by a TTTT motif and followed by a polyA tract of variable length. The right tRNA-like unit also contains a 16-bp sequence which is absent in the left one and appears to be located in the ancestral anticodon stem precisely at a position expected for a nuclear tRNA intron. According to this singular structure, we hypothesize that the TWIN: SINE family originated from an unprocessed polymerase III transcript containing two tRNA sequences. We suggest that some peculiar properties acquired by this dicistronic transcript, such as a polyA tail and a 3' stem-loop secondary structure, promote its retroposition by increasing its chances of being recognized by a reverse transcriptase encoded elsewhere in the C. pipiens genome.
Feschotte
C
C
Laboratoire Ecologie Moléculaire et Faculté Sciences et Techniques Côte-Basque, Université de Pau et des Pays de l'Adour, Pau, France.
Fourrier
N
N
Desmons
I
I
Mouchès
C
C
D000818:Animals; D001483:Base Sequence; D016384:Consensus Sequence; D003465:Culex / Q000235:genetics*; D004175:Diptera / Q000235:genetics; D004199:Disease Vectors; D019143:Evolution, Molecular; D018628:Gene Dosage; D016678:Genome; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D012343:RNA, Transfer / Q000235:genetics*; D012689:Sequence Homology, Nucleic Acid; D020086:Short Interspersed Nucleotide Elements / Q000235:genetics*
D012343:RNA, Transfer