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We have previously described a GC-rich human minisatellite, termed MsH42, which exists in two allelic forms, long and short. Here, we have identified a third allele of medium length and localized the MsH42 locus in the chromosome 15q25.1 inside an intron belonging to a gene of unknown function. The recombinogenic potential of the three alleles was assayed in vitro incubating pBR322-based constructs containing two copies of the minisatellite MsH42 with its flanking sequences, in the presence of rat testes nuclear extracts. This assay system was configured to monitor only reciprocal exchange type events and not gene conversion. All MsH42 allelic sequences enhanced intramolecular homologous recombination promoting high rates (approximately 76%) of equal crossover, the long allele showing the highest recombinogenic activity. Removal of the MsH42 long allele flanking sequences, which are identical in the three alleles, provoked a decrease in the enhancement of recombination and in the frequency of equal crossovers, suggesting that these sequences are important for the recombinogenic activity and for the correct pairing between homologous sequences. The occurrence of some complex recombination events within the minisatellite MsH42 suggests the existence of processes related to polymerase slippage and unwinding with reinvasion during the repair synthesis. Our findings point toward the existence of two distinct biochemical pathways for initiation and resolution of recombination at the minisatellite MsH42. Finally, the in vitro recombination system employed in this study could provide an approach to dissect processes of repetitive DNA instability and recombination.

A novel gene encoding a 6'-N-aminoglycoside acetyltransferase, aac(6')-In, has been cloned and sequenced from Citrobacter freundii 13996-19, a clinical isolate from Venezuela. This gene mediates resistance to amikacin, 2'-N-ethylnetilmicin, isepamicin, kanamycin, netilmicin, and tobramycin. The aac(6')-In gene is 573 nucleotides in length and encodes a putative protein of 190 amino acids. AAC(6')-In is most closely related to AAC(6')-Im and AAC(6')-Ie, demonstrating 64.4% and 62.3% similarity, respectively, at the protein level, suggesting these proteins share a common ancestor. The aac(6')-In flanking sequences demonstrated homology to integron- and transposon-related elements which are often found associated with resistance determinants. Hybridization studies performed with an intragenic probe specific for aac(6')-In indicate that this gene is prevalent within Venezuela but has not been observed outside of the country. Furthermore, the aac(6)-In gene was found in 10 different species of gram-negative bacteria.

Resistance to most of the antimicrobial agents in use today is present in Latin America as of this publication. Their underlying mechanisms are in place and an even more serious situation is foreseen in the years to come. Both nosocomial and common community-acquired infections have changed to require more complex ways of management. Although newer antibiotics take the place of the older ones, wiser and more restrictive usage of the currently available antibiotics is needed. This may be obtained through education and with the amplification of national and supranational networks of surveillance, which could anticipate trends in resistance.

Resistance to the endogenous antibiotic was studied in three actinomycetes that produce inhibitors of RNA polymerase. The three producers, Nocardia mediterranei (rifamycin producer), Streptomyces spectabilis (streptovaricin producer) and Streptomyces lydicus (streptolydigin producer), were each highly resistant to the antibiotic they produce (MIC greater than 200 micrograms ml-1) and in vivo RNA synthesis was also resistant. However, cross-resistance to the other RNA polymerase inhibitors was not found. Resistance to these antibiotics was due to target site modification, since the RNA polymerase enzymes of the three producing organisms were highly resistant in vitro to the corresponding antibiotic, and no antibiotic-inactivating enzymes were detected. A mutant was isolated from S. spectabilis which was sensitive to steptovaricin (its own product) and also showed an increased sensitivity to rifamycin and streptolydigin. This mutant had RNA polymerase which was extremely sensitive to the three antibiotics.

A study on the evolution of resistance to six aminoglycosides in Serratia as well as the relationship with the annual consumption of each drug in a hospital over the period 1974-1981 was carried out. The incidence of modifying enzymes and their genetical location was determined in 38 isolates. It was found that: The variations in the percentage of Sm clinical isolates showed no relationship with the consumption of Sm. Two different types of enzymes are involved in this resistance: AAD(3''): adenylyltransferase and APH(3''): phosphotransferase. The resistance to Nm, Km and Gm seems to be directly related with the continuous consumption of these drugs. In all the strains under study (Nm-Km)r was due to an APH(3')(5)I: phosphotransferase, Gmr to two types of acetyltransferases: AAC(3)I, only found in strains isolated before 1977 and AAC(3)II which predominates in 1981. After introducing Tm (1975) and Ak (1979) in our environment, there was an increase in the number of resistant strains. Two acetyltransferases with Tm-affinity were found: AAC(3)II and AAC(6')IV, the latter showing affinity for AKr. It was determined that five of these enzymes are plasmid-mediated. The genetical location of a sixth enzyme (AAC(6')IV) has not been clarified.