| Summary |
The purpose of this study is to prepare a map of the chromosome of Staphylococcus aureus strain 7-8 by determining the sequence of gene replication. Assumptions used in mapping the replication sequence of genes on the chromosome of Staphylococcus aureus were as follows: (1) genes on the chromosome were expected to replicate in sequence (Meselson and Stahl, 1958); (2) culture could be synchronized (Altenbern, 1966, 1968); (3) generation time of the synchronized culture could be determined; and (4) mutagen, nitrosoguanidine (NTG), was specific for DNA (Cerda-Olmedo, Hanawalt, and Guerola, 1968). Phenethyl alcohol was employed to synchronize the generation time of the culture of S. aureus strain 7-8. Synchronization was sufficient for the mapping procedure employed. The generation time was approximately 110 minutes under the conditions used. The action of NTG on the bacterial chromosome is incompletely understood. However, NTG repeatedly produced mutations at the same area on the chromosome of aureus strain 7-8 suggesting that NTG was adequate for use in replication sequence mapping by the chemical mutagen technique employed. The replication sequence mapping of aureus strain 7-8 was performed by determining time during one generation of a synchronized culture when treatment with the chemical mutagen, NTG, first caused iv an increase in number of mutants of the gene being studied. Temporal location of three genes: pantothenate, chloramphenicol and glutamic acid were found to be 40-45, 20-25 and 0-20 minutes respectively from the point of origin of chromosome synthesis. The direction of synthesis of the chromosome of aureus strain 7-8, as determined by the sequence of occurrence of the pantothenate and chloramphenicol loci, appeared to be opposite that implied by the results of Altenbem (1968) for another culture of aureus. We assumed that the point of initiation of chromosome synthesis was the same for all mutants of the same parent culture. This assumption was shown to be true for the culture of aureus used by Altenbem (1966, 1968) and for E. coli (Cerda-Olmedo et al., 1968). The point of initiation of chromosome synthesis by aureus strain 7-8 was different from that of the culture used by Altenbem (1966, 1968). This is not in agreement with the literature for E. coli. Cerda-Olmedo et al. (1968) reported that origin of replication of the three cultures of E. coli they tested was fixed at a determined position on the chromosome. The chloramphenicol and pantothenate loci were shown to be 20 map units apart on the chromosome of S. aureus strain 7-8. Altenbem (1968) reported these genes were the same distance apart on the chromosome of the S. aureus culture he used. |
