Duplex breaks in DNA as recombination initiators

Sigma elements are position-specific for many different yeast tRNA genes

We determined the DNA sequence of seventeen sigma elements and flanking regions in order to investigate the extent of the association between the yeast repetitive element, sigma, and tRNA genes. Fifteen of seventeen sigma elements analyzed begin at position -19 to -16 with respect to the 5' end of a tRNA-coding sequence. This region is close to the initiation point of tRNA gene transcription and contains a sequence which is modestly conserved for a number of tRNA genes. Two pairs of identical sigma elements occur as the long terminal repeats of a sequence which, together with flanking sigma elements, has the structural properties of a retrotransposon; this element has been named Ty3 (manuscript submitted). Hybridization analysis of yeast chromosomal DNA separated by orthogonal field alternation gel electrophoresis (OFAGE) showed that Ty3 and isolated sigma elements are distributed over many chromosomes in the yeast genome.

A restriction and modification model for the initiation and control of recombination in Neurospora

It is hypothesized that the products of Neurosporarec+genes mask recombinators such ascogby modifying DNA and that unmodified recombinators act as recognition sites for an endonuclease with scission properties like those of the type I restriction enzymes found inE. coli. These cut the DNA in both strands at some variable distance from a recognition site. Repair of a two strand gap initiated in this way would require DNA synthesis using the information contained in the homologous DNA duplex, leading to gene conversion. Crossing over could follow from resolution of two Holliday structures formed during gap repair. The hypothesis explains the polarity in the frequency of conversion events across genetic loci, the observation that chromosomes carrying recombinators are more often converted than is the homologue, and how recombinators can initiate conversion at a distance, as suggested by the pattern of conversion events in thehis-3locus in crosses heterozygous for the translocation TM429.

Selective telomere activation and the control of antigen gene expression in trypanosomes

African trypanosomes escape the immune defence of their mammalian host by changing their antigenic surface coat. Antigenic variation occurs through differential gene activation: only one antigen gene is transcribed at a time, among a large collection of specific sequences. This transcription always takes place in a telomere, but it seems that different telomeres can be used alternatively as the gene expression site. Since the trypanosome genome is made up of numerous chromosomes, it would appear that a highly selective process allows the activation of only one telomere at a time. This process seems linked to the differential inactivation of a peculiar telomeric DNA modification system. Two mechanisms allow antigen genes to be expressed. First, a gene copy can be inserted in the expression site by replacing the formerly expressed gene. This is due to gene conversion, whose extent can vary considerably, according to the degree of homology between the recombining partners. The second mechanism involves the activation of another telomere along with deactivation of the telomere containing the previously expressed gene. This form of activation can occur without apparent DNA rearrangement. The alternate use of these mechanisms leads to rapid changes in the antigen gene repertoire, due to gain and loss of different sequences, and to alteration of their activation rate.