Spontaneous insertions into cosmid vector: a warning

During the course of characterization of a human genomic library we found that some of the selected pNNL cosmid clones carried only very short inserts. In addition, several clones that did contain full-length inserts were found to be unstable and generated repeated deletions of various portions of their inserts. Moreover, all the clones examined displayed rearrangements in the vector portions. The rearranged clones that were characterized by restriction mapping were found to have deletions starting between ClaI and HindIII in the region of the cos segment of the pNNL vector used in the construction of the library. We determined the nucleotide sequence of the 1300-bp EcoRI-PvuII segment located in the cos region, and by the computer search we found that it contains Escherichia coli insertion elements IS1.