Preface

It has been a little over two decades since the stable propagation of 100 kb-sized DNA in bacteria by Drs. Nancy Shepherd and Nat Sternberg using the phage P1 packaging system. The Bacterial Artificial Chromosome (BAC) system was developed soon after by Drs. Hiroaki Shizuya, Bruce Birren, Ung-Jin Kim, Melvin Simon and colleagues. Genomic DNA libraries are easier to construct using electroporation, instead of P1 packaging, and clones can propagate DNA of much larger size using the BAC system. As a consequence, BACs became very popular among researchers in the genome community and Drs. Pieter de Jong, Kazutoyo Osoegawa, Chris Amemiya and their colleagues generated a series of genomic DNA libraries from several vertebrate organisms that are not only of much higher coverage of their respective genomes but also comprised of clones that had DNA inserts of larger average size. These libraries played important roles in the assembly of genome sequences of several vertebrate organisms including the human, mapping genes and genetic markers on chromosomes, and serving as useful tools in comparative genomics studies of related species. A chapter representative of such applications of BAC libraries is included in this book.

The past decade witnessed the wide spread use of clones from BAC libraries of numerous organisms for functional studies. The large insert DNA size and easy maneuverability of that DNA in bacteria has contributed to the growing popularity of BACs in transgenic animal studies. The realization that many control elements of genes important during vertebrate development are actually located at large distances along the DNA from the coding sequences of the gene have made BACs increasingly indispensable for studies of developmentally regulated genes using transgenic animals. A different area of interest arose from the same attractive features of BACs, and relates to their use as vectors for cloning the very large genomes of several DNA viruses. Faithful propagation and easy mutational analyses of the BAC-viral DNA in bacteria allowed rapid assignment of function(s) to the numerous open reading frames in the viral genome when that BAC-viral DNA was reintroduced into permissive hosts for a productive infection. Several chapters of this book illustrate the variety of applications in this area.

Several new technologies have been developed to alter sequences in BAC DNA within its bacterial host. While all of these methods utilize DNA recombination of some sort, the more widely used ones require re-introducing homologous

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recombination function of E.coli or phage λ back into the severely recombination deficient host. This book also contains a couple of chapters illustrating the usefulness of BACs in functionally mapping gene regulatory elements. In this context the recent demonstration by Dr. Koichi Kawakami and colleagues that the vertebrate transposon system Tol2 can be re-engineered to facilitate integration of BAC DNA into the chromosomes of zebrafish and mice is likely to accelerate the use of BACs in a variety of studies with transgenic animals.

This book focuses on the numerous applications of Bacterial Artificial Chromosomes (BACs) in a variety of studies. The topics reviewed range from using BAC libraries as resources for marsupial and monotreme gene mapping and comparative genomic studies, to using BACs as vehicles for maintaining the large infectious DNA genomes of viruses. The large size of the insert DNA in BACs and the ease of engineering mutations in that DNA within the bacterial host, allowed manipulating the BAC-viral DNA of Varicella-Zoster Virus. Other reviews include the maintenance and suitable expression of foreign genes from a Baculovirus genome, including protein complexes, from the BAC-viral DNA and generating vaccines from BAC-viral DNA genomes of Marek's disease virus. Production of multi-purpose BAC clones in the novel Bacillus subtilis host is described, along with chapters that illustrate the use of BAC transgenic animals to address important issues of gene regulation in vertebrates, such as functionally identifying novel cis-acting distal gene regulatory sequences.

> **Pradeep K. Chatterjee** Associate Professor Biomedical/Biotechnology Research Institute North Carolina Central University, Durham USA
