Abundant pseudogenes are a feature of mammalian genomes. Processed pseudogenes (PPs) are reverse transcribed from mRNAs. Recent molecular biological studies show that mammalian long interspersed element 1 (L1)-encoded proteins may have been involved in PP reverse transcription. Here, we present the first comprehensive analysis of human PPs using all known human genes as queries. The human genome was queried and 3,664 candidate PPs were identified. The most abundant were copies of genes encoding keratin 18, glyceraldehyde-3-phosphate dehydrogenase and ribosomal protein L21. A simple method was developed to estimate the level of nucleotide substitutions (and therefore the age) of PPs. A Poisson-like age distribution was obtained with a mean age close to that of the Alu repeats, the predominant human short interspersed elements. These data suggest a nearly simultaneous burst of PP and Alu formation in the genomes of ancestral primates. The peak period of amplification of these two distinct retrotransposons was estimated to be 40-50 million years ago. Concordant amplification of certain L1 subfamilies with PPs and Alus was observed. We suggest that a burst of formation of PPs and Alus occurred in the genome of ancestral primates. One possible mechanism is that proteins encoded by members of particular L1 subfamilies acquired an enhanced ability to recognize cytosolic RNAs in trans.

Many SINEs and LINEs have been characterized to date, and examples of the SINE and LINE pair that have the same 3′ end sequence have also increased. We report the phylogenetic relationships of nearly all known LINEs from which SINEs are derived, including a new example of a SINE/LINE pair identified in the salmon genome. We also use several biological examples to discuss the impact and significance of SINEs and LINEs in the evolution of vertebrate genomes.

Short interspersed repetitive elements (SINEs) are a type of retroposon, being members of a class of informational molecules that are amplified via cDNA intermediates and flow back into the host genome. In contrast to retroviruses and retrotransposons, SINEs do not encode the enzymes required for their amplification, such as reverse transcriptases, so they are presumed to borrow these enzymes from other sources. In the present study, we isolated a family of long interspersed repetitive elements (LINEs) from the turtle genome. The sequence of this family was found to be very similar to those of the avian CR1 family. To our surprise, the sequence at the 39 end of the LINE in the turtle genome was nearly identical to that of a family of tortoise SINEs. Since CR1-like LINEs are widespread in birds and in many other reptiles, including the turtle, and since the tortoise SINEs are only found in vertical-necked turtles, it seems possible that the sequence at the 39 end of the tortoise SINEs might have been generated by recombination with the CR1-like LINE in a common ancestor of vertical-necked turtles, after the divergence of side-necked turtles. We extended our observations to show that the 39-end sequences of families of several tRNA-derived SINEs, such as the salmonid HpaI family, the tobacco TS family, and the salmon SmaI family, might have originated from the respective LINEs. Since it appears reasonable that the recognition sites of LINEs for reverse transcriptase are located within their 39-end sequences, these results provide the basis for a general scheme for the mechanism by which SINEs might acquire retropositional activity. We propose here that tRNA-derived SINEs might have been generated by a recombination event in which a strong-stop DNA with a primer tRNA, which is an intermediate in the replication of certain retroviruses and long terminal repeat retrotransposons, was directly integrated at the 39 end of a LINE.

We have identified a new superfamily of vertebrate short interspersed repetitive elements (SINEs), designated V-SINEs, that are widespread in fishes and frogs. Each V-SINE includes a central conserved domain preceded by a 5′-end tRNA-related region and followed by a potentially recombinogenic (TG)_n tract, with a 3′ tail derived from the 3′ untranslated region (UTR) of the corresponding partner long interspersed repetitive element (LINE) that encodes a functional reverse transcriptase. The central domain is strongly conserved and is even found in SINEs in the lamprey genome, suggesting that V-SINEs might be ∼550 Myr old or older in view of the timing of divergence of the lamprey lineage from the bony fish lineage. The central conserved domain might have been subject to some form of positive selection. Although the contemporary 3′ tails of V-SINEs differ from one another, it is possible that the original 3′ tail might have been replaced, via recombination, by the 3′ tails of more active partner LINEs, thereby retaining retropositional activity and the ability to survive for long periods on the evolutionary time scale. It seems plausible that V-SINEs may have some function(s) that have been maintained by the coevolution of SINEs and LINEs during the evolution of vertebrates.[The sequences reported in this paper have been deposited in the DDBJ/GenBank database under accession nos.AB072981–AB073004. Supplemental figures are available online athttp://www.genome.org.]