RESUMO
Spontaneous tandem chromosomal duplications are common in populations of Escherichia coli and Salmonella typhimurium. They range in frequency for a given locus from 10(-2) to 10(-4) and probably form by RecA-dependent unequal sister strand exchanges between repetitive sequences in direct order. Certain duplications have been observed previously to confer a growth advantage under specific selective conditions. Tandem chromosomal duplications are unstable and are lost at high frequencies, representing a readily reversible source of genomic variation. Six copies of a small mobile genetic element IS200 are evenly distributed around the chromosome of S. typhimurium strain LT2. A survey of 120 independent chromosomal duplications (20 for each of six loci) revealed that recombination between IS200 elements accounted for the majority of the duplications isolated for three of the loci tested. Duplications of the his operon were almost exclusively due to recombination between repeated IS200 elements. These data add further support to the idea that mobile genetic elements provide sequence repeats that play an important role in recombinational chromosome rearrangements, which may contribute to adaptation of bacteria to stressful conditions.
Assuntos
Aberrações Cromossômicas , Cromossomos Bacterianos , Elementos de DNA Transponíveis , Recombinação Genética , Salmonella typhimurium/genética , Mapeamento por RestriçãoAssuntos
DNA Fúngico/genética , DNA Ribossômico/genética , Genes Fúngicos , Neurospora crassa/genética , Neurospora/genética , Pseudogenes , RNA Fúngico/genética , RNA Ribossômico 5S/genética , RNA Ribossômico/genética , DNA (Citosina-5-)-Metiltransferases/fisiologia , DNA Fúngico/metabolismo , DNA Ribossômico/metabolismo , Proteínas Fúngicas/fisiologia , Metilação , Família Multigênica , Neurospora crassa/fisiologiaRESUMO
Introduction of DNA into Neurospora crassa can lead to sequence instability in the sexual phase of the life cycle. Sequence instability was investigated by using a set of strains transformed with single copies of a plasmid including host sequences, Neurospora sequences deleted from the host genome, and foreign sequences. The sequences already represented in the host were rearranged at high frequency in a cross. In general, both elements of the duplication, that from the plasmid and that from the host, became rearranged, whether or not they were linked. Unique sequences were left unaltered. Cytosine residues in the rearranged sequences typically became methylated de novo. Results from tetrad analyses indicated that the rearrangements occur before meiosis, during a stage between fertilization and karyogamy. We suggest that this previously unrecognized genetic process, RIP (rearrangement induced premeiotically), may contribute diversity for evolution and also maintain the gross organization of the genome.
