Understanding of the molecular system for DNA delivery into eucaryotic cells, a key to human DNA therapy, remains obscure. To understand this system, we undertook a study using the Saccharomyces cerevisiae model into which DNA delivery is easily assessed through competence (transformability) and for which all nonessential gene mutants (about 5000 strains) are available. We analyzed the competence of each of these mutants and identified three low-competence mutants, i.e., sin3Delta, she4Delta, and arc18Delta, and three high-competence mutants, i.e., pde2Delta, spf1Delta, and pmr1Delta. Through further studies using the six mutants, we concluded that the Arp2/3 activation machinery involving the Myo3/5p, Vrp1p, Las17p, Pan1p, and Arp2/3 complex is crucial to delivery (competence), and that high cAMP enhances competence via protein kinase A installing Tpk3p. We also propose that DNA is taken up via an endocytosis-like event, being at least partially different from well-known endocytosis. (C) 2004 Elsevier Inc. All rights reserved.