A plasmid of about 140 megadaltons has been associated with the invasiveness of Shigella flexneri. Upon subculturing in liquid media of fully virulent isolates of Shigella flexneri 2a YSH6000, which contains only a 230-kilobase-pair (kbp) plasmid in addition to 3.3-and 4.2-kbp cryptic plasmids characteristic to all S. flexneri strains, loss of invasiveness, loss of Congo red binding activity (Pcr), and complete loss of, or a deletion, or even a single-site IS insertion in the plasmid occurred simultaneously. This was ascribed to the fact that, once a noninvasive Pcrcell has emerged, it overgrows the wild type as a consequence of its selective advantage in artificial media. A deletion map of the 230-kbp plasmid was made by analyzing Sall digests of 39 deletion derivatives plus 1 formed by insertion of an IS1-like element in independently isolated, noninvasive Pcrmutants. Of 39 deletion derivatives, 16 belonged to a single type, and 6 belonged to another, suggesting deletion hot spots. The deletion map was confirmed and extended by analyzing 359 Sail-generated partial digests of the wild-type plasmid cloned into pBR322. Three copies of IS1-like elements were found on three different Sall fragments by Southern hybridization. Segments required for the Pcr+ phenotype seemed to occur at several different locations in the plasmid. Each of 28 representative Pcrmutants were negative by the Sereny test. Hence, many, or possibly all, Pcr determinants were required for full virulence.

Expression of the umuDC operon is required for UV and most chemical mutagenesis in Escherichia coli. The DNA which can restore UV mutability to a umuD44 strain and to a umuC122::Tn5 strain of E. coli has been cloned from Salmonella typhimurium TA1538. DNA sequence analysis indicated that the cloned DNA potentially encoded proteins with calculated molecular weights of 15,523 and 47,726 and was an analog of the E. coli umuDC operon. We have termed this cloned DNA the samAB (for Salmonella mutagenesis) operon and tentatively referred to the umuDC operon of S. typhimurium LT2 (C. M. Smith, W. H. Koch, S. B. Franklin, P. L. Foster, T. A. Cebula, and E. Eisenstadt, J. Bacteriol. 172:4964-4978, 1990; S. M. Thomas, H. M.Crowne, S. C. Pidsley, and S. G. Sedgwick, J. Bacteriol. 172:4979-4987, 1990) as the umuDCST operon. The samAB operon is 40% diverged from the umuDCST operon at the nucleotide level. Among five umuDC-like operons so far sequenced, i.e., the samAB, umuDCST, mucAB, impAB, and E. coli umuDC operons, the samAB operon shows the highest similarity to the impAB operon of TP110 plasmid while the umuDCST operon shows the highest similarity to the E. coli umuDC operon. Southern hybridization experiments indicated that (i) S. typhimurium LT2 and TA1538 had both the samAB and the umuDCST operons and (ii) the samAB operon was located in a 60-MDa cryptic plasmid. The umuDCsT operon is present in the chromosome. The presence of the two homologous but different umuDC operons may be involved in the poor mutability of S. typhimurium by UV and chemical mutagens.

To localize the oral primary somatosensory cortex, we measured somatosensory-evoked fields for the lip, gingiva, and tongue in six healthy subjects. The latency of the first peak of the posterior-oriented current in the contralateral hemisphere was 50.9 +/- 8.3 ms for the gingiva, significantly shorter than those for the lip and tongue peaks. The equivalent current dipole was localized on the central sulcus. The gingival dipole was localized significantly inferior to the lip dipole but not different from the tongue dipole. The moment of the gingival dipole was significantly smaller than that of the lip dipole but not different from that of the tongue dipole. Differences in the above parameters were negligible between the left and right, anterior and posterior, and upper and lower locations within the same organ, except that the dipole location for the anterior upper tongue was significantly inferior to that for the lower tongue.