Cover: The ability to generate mutant alleles and screen them for informative phenotypes is a fundamental approach for investigating gene function. Recombination cloning strategies have been devised that allow large collection of genes to be seamlessly transferred to different expression formats, yielding constructs in which genes are juxtaposed to novel regulatory elements or fused to epitope tags. In this issue, Khalil et al describe a mutant library construction technique that allows these powerful recombination cloning strategies to be used for processing large collections of gene mutations, enabling recombineering to be applied to mutant screening procedures. The figure displays a phenotype associated with a mutant isolated through this procedure. DBF4 is a yeast gene required to initiate DNA replication. A single temperature sensitive allele, dbf4-1, has been described. At the non-permissive temperature dbf4-1 cells (the two cells on the right side of the figure) arrest in the cell cycle with a large budded morphology, but undergo a mitotic catastrophe in which the mitotic spindle (yellow-green) extends, segregating unreplicated chromosomes (blue). As one application of their method, the authors isolated additional temperature sensitive dbf4 alleles. One of these new alleles, dbf4-C22 (the two cells on the left), also arrests with a large budded morphology, but is proficient for maintaining pre-anaphase spindle structure. The authors suggest their technique is generally applicable to isolating informative mutants under a range of experimental conditions. See Khalil et al, for more information (Nucleic Acids Res. (2007) 35, e104).
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