The behavior of chromosomal inversions in Escherichia coli depends upon the region they affect. Regions flanking the replication terminus have been termed nondivisible zones (NDZ) because inversions ending in the region were either deleterious or not feasible. This regional phenomenon is further analyzed here. Thirty segments distributed between 23 and 29 min on the chromosome map have been submitted to an inversion test. Twenty-five segments either became deleterious when inverted or were noninvertible, but five segments tolerated inversion. The involvement of polar replication pause sites in this distribution was investigated. The results suggest that the Tus/pause site system may forbid some inversion events, but that other constraints to inversion, unrelated to this system, exist. Our current model for deleterious inversions is that the segments involved carry polar sequences acting in concert with other polar sequences located outside the segments. The observed patchwork of refractory and tolerant segments supports the existence of several NDZs in the 23- to 29-min region. Microscopic observations revealed that deleterious inversions are associated with high frequencies of abnormal nucleoid structure and distribution. Combined with other information, the data suggest that NDZs participate in the organization of the terminal domain of the nucleoid.

The E. coli chromosome replication arms are polarized by motifs such as RRNAGGGS oligomers, found preferentially on leading strands. Their skew increases regularly from the origin to dif (the site in the center of the terminus where chromosome dimer resolution occurs), to reach a value of 90% near dif. Convergent information indicates that polarization in opposite directions from the dif region controls tightly the activity of dif, probably by orienting mobilization of the terminus at cell division. Another example of polarization is the presence, in the region peripheral to the terminus, of small non-divisible zones whose inversion interferes with spatial separation of sister nucleoids. The two phenomena may contribute to the organization of the Ter macrodomain.

RAPD analysis was used to estimate the genetic diversity in an Iberian imperial eagle (Aquila adalberti) population, one of the most threatened bird species in the world. Forty-five of 60 arbitrarily designed primers amplified 614 loci in 25 individual eagles, 59.7% of which were polymorphic. In contrast to the traditional allozyme analysis performed in a previous study, the RAPD method has revealed a high level of heterozygosity in this species (H = 0.267+/-0.008). The genetic distances estimated between 25 eagles can serve to establish more adequate mating in order to preserve genetic variability. Conservation efforts being carried out in Spain in this species might be successful based on the results obtained in the present work.