N Bensaadi-Merchermek | Search PubMed

PubMed1995

Characterization of the unlinked 16S rDNA and 23S-5S rRNA operon of Wolbachia pipientis, a prokaryotic parasite of insect gonads.

The rRNA-encoding genes (rDNAs) have been cloned and characterized from Wolbachia pipientis (Wp), the gonadial bacteria-like parasite of the mosquito Culex pipiens (Cp) and the moth Ephestia cautella (Ec). In Wp from both insect species the rDNAs are organized in a way which appears to be very unusual. The rRNAs are encoded by two unlinked transcription units, each present in a single copy per genome. One contains the 16S rDNA only, while the other is an operon encoding both the 23S and 5S rDNAs. Each transcription unit contains two putative upstream promoters, and downstream a Rho-independent terminator. The 16S rDNA, as well as the 23S-5S rRNA operon are not linked to any tRNA-encoding sequence and lack the antitermination boxes which are usually present immediately downstream from eubacterial promoters of rDNAs. Wp infecting Ec and Cp are highly similar taking as criteria the rDNAs and their flanking sequences. However, it clearly appears that each insect species harbours a different and specific Wp strain, or even subspecies. Phylogenetic relationships deduced from the complete sequences of their rDNAs undoubtedly confirm that Wp from Cp and Ec belong to the alpha-group of Proteobacteria, and are closely related to the Rickettsia.

Animals↗

PubMed1994

Mosquito transposable elements.

Most of the transposons so far characterized from mosquito genomes are retroelements which seem to be distributed worldwide. The Juan transposons constitute a family of non-LTR retroelements, or LINE-retroposons, which are dispersed in the genomes of several mosquito species. Three different Juan subfamilies have been characterized, each being amplified in the genomes of many strains, if not all, of a given mosquito species. These subfamilies have been designated respectively Juan-C in Culex pipiens, Juan-Ct in Culex tarsalis and Juan-A in Aedes aegypti. A large number of the Juan retroposons which are amplified in the mosquito genomes are apparently full-length copies and potentially encode the enzymes necessary for their transposition, a nucleic acid binding protein and a reverse transcriptase. However, these complete Juan copies seem to be most frequently transcriptionally silent in insects reared under laboratory conditions. A few of them are transcribed in C. pipiens cells grown in vitro, but from an external promoter, the Juan-C specific RNA being fused to an upstream RNA sequence. Therefore, the transcription of Juan retroposons seems to depend on external promoters which are most frequently inactive. The occurrence and distribution of Juan retroposon subfamilies among mosquito species do not reflect the phylogeny of these species. Furthermore, complete Juan-C and Juan-A copies which are reiterated in strains collected from regions covering different continents are nearly identical. Juan-C copies belonging to geographically different C. pipiens strains display low levels of divergence between their nucleotide sequences and many of the mutations which have occurred among these copies do not alter their coding potential. These results indicate that the Juan retroposons occur as homogeneous subfamilies distributed worldwide and that selective constraints against amino acid change have been acting recently on these elements, despite the fact that they are now highly repeated through mosquito genomes. Therefore, Juan transposons have most probably been recently amplified in mosquito genomes. Each subfamily may have been amplified from one master element present in a unique population which has since spread worldwide. Alternatively, this amplification may have arisen in many mosquito populations, but from highly conserved master elements submitted to selection pressures. Horizontal transfers between species may also have contributed to the spread of these transposons.

Aedes↗