The genome size and a partial physical and genetic map have been defined for the phage group II Staphylococcus aureus Ps55. The genome size was estimated to be 2,771 kb by pulsed-field gel electrophoresis (PFGE) using the restriction enzymes SmaI, CspI, and SgrAI. The Ps55 chromosome map was constructed by transduction of auxotrophic and cryptic transposon insertions, with known genetic and physical locations in S. aureus NCTC 8325, into the Ps55 background. PFGE and DNA hybridization analysis were used to detect the location of the transposon in Ps55. Ps55 restriction fragments were then ordered on the basis of genetic conservation between the two strains. Cloned DNA probes containing the lactose operon (lac) and genes encoding staphylococcal protein A (spa), gamma hemolysin (hlg), and coagulase (coa) were also located on the map by PFGE and hybridization analysis. This methodology enabled a direct comparison of chromosomal organization between NCTC 8325 and Ps55 strains. The chromosome size, gene order, and some of the restriction sites are conserved between the two phage group strains.A physical map of a chromosome consists of an ordered set of restriction fragments that places genomic sequence in the order it exists inside the cell. The resolution of chromosome structure can vary from a macrorestriction map consisting of only a few large, restriction fragments (10, 14, 17, 37) to thousands of small clones of ordered chromosomal DNA (4, 13) to a complete nucleotide sequence (11). The advantages of a physical map over a genetic map are severalfold. Physical maps are faster to construct and require considerably less laborintensive methods. Methods in genetic mapping, such as conjugation, transformation, transduction, and mutagenesis, can be bypassed in physical map construction. Many prokaryotic physical maps have been constructed as a starting point to characterize the chromosome because methods of genetic analysis were underdeveloped (1,2,5,14,15,17,43,47). Other physical maps were constructed to confirm previously established genetic maps (10,28,46).With the establishment of long-range genomic mapping techniques (20,36), there has been a recent surge in the field of prokaryotic genomic mapping. With these maps constructed, a preliminary assessment of chromosomal diversity among the prokaryotes may become possible. This assessment of chromosomal diversity has already been examined for species in the genus Neisseria (8), where a newly constructed Neisseria meningitidis map was compared with a previously constructed map of a gonococcal strain. Smith and Condemine (36) stated that "technology needs to be developed to simplify map construction by using maps that already exist for related organisms." This project outlines one such technology that was applied to Staphylococcus aureus.The species is divided into five phage groups on the basis of typing patterns produced on infection with staphylococcal phages (23,32,33,42,45,48). A detailed genetic and physical map has been established in the phage group III S. aure...