A chromosome containing HOT1 preferentially receives information during mitotic interchromosomal gene conversion.

Abstract

The recombination-stimulating sequence, HOT1, is derived from yeast ribosomal DNA and corresponds to the sequences required for promotion of transcription by RNA polymerase I. The effect of HOT1 on mitotic interchromosomal gene conversion was examined in diploid strains carrying his4 heteroalleles. When HOT1 is inserted adjacent to both copies of HIS4, the frequency of His+ recombinants is increased approximately 10-fold. When HOT1 is present on only one of the two homologs, recombination is enhanced and the his4 gene on the HOT1-containing chromosome is preferentially converted. In both pairs of his4 heteroalleles examined, HOT1 stimulates conversion of the his4 mutation which is further from the site of HOT1 insertion more than it stimulates conversion of the HOT1-proximal his4 allele. Compared to recombinants isolated from control strains that lack HOT1, HOT1-promoted His+ recombinants are more often homozygous for sequences distal to HIS4. The preferential conversion of sequences on the HOT1-containing chromosome is consistent with the double-strand-gap repair model of recombination and suggests that HOT1-promoted gene conversion initiates with a double-strand break in HOT1-adjacent sequences.

Transcription-Associated Genome Instability

“…Crossover events were increased when the donor sequence was highly transcribed whereas high transcription of the recipient sequence enhanced gene conversion events. This result is in agreement with the pattern of gene conversion events analyzed in the HOT1 -dependent system in which the highly transcribed repeat was also the favored acceptor of genetic information . Since the copy undergoing the initiation event generally acts as the recipient allele during mitotic recombination, these results suggest that transcription by RNAPII increases the occurrence of initiating events.…”

“…Subsequently, HOT1 activity was shown to rely on its ability to promote transcription by RNAPI − (Figure ). The impact of HOT1 on recombination between two homologous chromosomes in diploid yeast cells has been extensively studied to gain insight into the mechanisms of TAR . Recombination was assayed between two different his4 alleles, one in each copy of chromosome III, which were marked with the TRP1 and LEU2 genes for discrimination purposes.…”

The Meiosis-Specific Hop2 Protein of S. cerevisiae Ensures Synapsis between Homologous Chromosomes

“…Strains YAB45, YAB46, ogy, with only long-range homology serving to satisfy YAB49, and YAB51 are isogenic and derived by mating RLK1-3C, or Hop2p that homologs have been found. Another possia transformant thereof, to K144, or a transformant thereof (Voelkelble function for Hop2p is suggested by observations of Meiman and Roeder, 1990). short stretches of SC between nonhomologous chromo-To sporulate cells for cytological analyses and physical assays of recombination, cells were pregrown in YPA (Bishop et al, 1992) somes during zygotene in wild-type plants (reviewed by to a concentration of ‫2ف‬ ϫ 10 7 cells/ml and then washed and resus-Carpenter, 1987).…”

Tandem repeats in extrachromosomal ribosomal DNA of Dictyostelium discoideum, resulting from chromosomal mutations.

“…One of these fragments corresponds to the transcription initiation site of RNA polymerase I (VOELKEL-MEIMAN, KEIL and ROEDER 1987), and has been shown to stimulate recombination when promoting transcription (STEWART and ROEDER 1989). VOELKEL-MEIMAN and ROEDER (1990) found that HOT2 stimulates recombination of a distant marker more than a nearby marker in the same gene. LIU and WANG (1 987) have postulated that negative and positive supercoiling behind and in front of the transcription complex during transcription causes the build up of torsional stress which is relieved by topoisomerases.…”