Cosmid libraries are important tools for molecular analysis of plant genes because of their useful properties. Cosmids carry the cos site from the bacteriophage lambda (1–4); therefore, cosmid libraries have the advantage of being able to be packaged in vitro using commercially available highly efficient packaging extracts, just as lambda libraries can. The high efficiency of in vitro lambda packaging allows a cosmid library with large inserts to be more easily constructed than a simple plasmid library, which relies on transformation of competent Escherichia coli cells. In addition, cosmids contain an origin of replication for propagation in E. coli and possibly other bacteria. Since cosmids are propagated as plasmids, not as lambda phages, the genes for lambda reproduction are not needed; therefore, a cosmid vector can be much smaller than a lambda vector (Fig. 1). Since lambda phage packages about 37–53 kb of DNA (5), cosmids can have relatively large inserts of more than 40 kb, depending on vector sizes. Another advantage of cosmids is the ease with which to prepare the DNA because plasmid DNA is much more readily isolated than lambda phage DNA. Finally, the fact that cosmids are plasmids allows the recombinant molecules to be transferred from one bacterium to another through conjugation and from Agrobacterium to plant cells, as long as the necessary cis elements are present. Therefore, cosmid libraries offer a unique combination of very useful features, which make them important for many different molecular studies, including those of plant genes.

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