Transposon mutagenesis can be used to map a mutation in the genome sequence FIGURE 5-38 The insertion of a transposon inserts a mutation into a gene of unknown position and function. The segment next to the transposon is replicated, sequenced, and matched to a segment in the complete genome sequence. Wild type cell carrying a resistance allele or other selectable marker is introduced by transfor- mation into bacterial recipients that have no active transposons. The transposons insert more or less randomly, and any that land in the middle of a gene cause a mutation. A subset of all mutants obtained will have phenotypes relevant to the bacterial process under study, and these phenotypes become the focus of the analysis. The beauty of inserting transposons is that, because their sequence is known, the mutant gene can be located and sequenced. DNA replication primers are cre ated that match the known sequence of the transposon (see Chapter 10). These primers are used to initiate a sequencing analysis that proceeds ourward from the transposon into the surrounding gene. The short sequence obtained can then be fed into a computer and compared with the complete genome sequence. From this analysis, the position of the gene and its full sequence are obtained. The func- tion of a homolog of this gene might already have been isms. Hence, you can see that this approach (like that introduced in Chapter 4) is another way of uniting mutant phenotype with map position and potential func- tion. Figure 5-38 summarizes the approach. As an aside in closing, it is interesting that many of the historical experiments revealing the circularity of bacterial and plasmid genomes coincided with the publication and popularization of J. R. R. Tolkien's The Lord of the Rings. Conse- quently, a review of bacterial genetics at that time led off with the following quo- tation from the trilogy: Transposon- Mutant phenatype induced by transposon insertion in other organ Primed synthesis Whole gene identified from genome sequence One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them.