No abstract

In ciliated protozoa, development of the macronucleus from a copy of the germ line micronucleus involves elimination of a large number of sequences by DNA splicing akin to precise excision of transposons. The known examples of such internal eliminated sequences (lESs) are not repetitive. The telomere-bearing elements (TBEls) of Oxytricha fallax are a family of transposons. We show that two particular TBEls are also IESs.TBEI-1 and TBE1-2 disrupt a micronuclear region that codes for macronuclear DNA. A variety of tests indicates that each TBE1 and one copy of the flanking target repeat is absent from most, if not all, molecules of the macronuclear DNA, as if the TBEls were precisely excised during macronuclear development. Three alternative explanations for the absence of TBEI-1 and TBE1-2 from the macronuclear DNA were tested. First, because two other highly homologous versions of that DNA are also found in the macronucleus, recombination between versions during or after macronuclear development could have bypassed the elements. Recombination in the regions flanking the elements was not detected. Second, micronuclear DNA blots show no evidence of a micronuclear counterpart of the macronuclear region that lacks TBEI-1. Third, TBE1-2 was demonstrated in two sexually independent cell lines. This shows that it pre-existed in the germ line, as opposed to having transposed into the micronuclear DNA subsequent to the generation of the macronucleus of the vegetative line that is usually studied. We conclude that TBEI-1 and TBE1-2, and possibly many of the other -1900 micronucleus-limited TBEls are excised as IESs during macronuclear development. These transposons appear to enjoy the luxury of relaxed constraints on family expansion, because they are removed from the genome before it is expressed. We discuss the possibility that all IESs are transposon-derived, that all are excised by transposition machinery, and that linear excision products are early intermediates in transposition. Ciliated protozoa interpose genomic reorganization, including massive sequence elimination, between the inheritance of germ line information and its expression. This genomic editing takes place in the developing macronucleus at the beginning of the sexual life of the organism, immediately after mating, meiosis, and crossfertilization. While the macronucleus is developing from one mitotic product of the zygotic nucleus, another mitotic product is set aside as the new germ line micronucleus. Once equipped with a new micronucleus and a mature new macronucleus, the exconjugant grows and divides clonally, duplicating the micronucleus and macronucleus of each vegetative cell cycle. The resultant clonal cell line is called a karyonide. The mature macronucleus is often considered a somatic nucleus, because most, if not all, gene expression occurs from its genome, whereas the micronucleus is silent and serves 1Current address:

Two new chromone derivatives have been identified which possess oral anti-allergic activity in the rat PCA model of immediate hypersensitivity. They may have a wider spectrum of anti-allergic activity than disodium cromoglycate (SCG) since they are effective in in vitro tests involving sensitized basophils, in which SCG is inactive. Both compounds, when given orally, provide relief from experimental and clinical asthma in man.

No abstract

DNA rearrangements in the form of deletions and duplications are found within and near integrated simian virus 40 (SV40) DNA in nonpermissive cell lines. We have found that rearrangements also occur frequently with integrated pSV2neo plasmid DNA. pSV2neo contains the entire SV40 control region, including the origin of replication, both promoters, and the enhancer sequences. Linearized plasmid DNA was electroporated into Xl, an SV40-transformed mouse cell line that expresses SV40 large T antigen (T Ag) and shows very frequent rearrangements at the SV40 locus, and into LMtk-, a spontaneously transformed mouse cell line that contains no SV40 DNA. Stability was analyzed by subcloning G-418-resistant clones and examining specific DNA fragments for alterations in size. Five independent Xl clones containing pSV2neo DNA were unstable at both the neo locus and the T Ag locus. By contrast, four Xl clones containing mutants of pSV2neo with small deletions in the SV40 core origin and three Xl clones containing a different neo plasmid lacking SV40 sequences were stable at the neo locus, although they were still unstable at the T Ag locus. Surprisingly, five independent LMtkclones containing pSV2neo DNA were unstable at the neo locus. LMtk-clones containing origin deletion mutants were more stable but were not as stable as the Xl clones containing the same plasmid DNA. We conclude that the SV40 origin of replication and early control region are sufficient viral components for the genomic instability at sites of SV40 integration and that SV40 T Ag is not required.