In five species of squid, varying in life-style from fast-swimming pelagic predators to sluggish benthic forms, the circular muscle of the mantle was found to be metabolically and structurally differentiated into inner, middle, and outer zones. In the middle zone, mitochondrial abundance and the ratios of oxidative to glycolytic enzyme activities were low. This zone was sandwiched between thinner bands of muscle lining both the inner and outer edges of the mantle. In these bands, mitochondrial abundance and the ratios of oxidative to glycolytic enzyme activities were high. It is proposed that this metabolic differentiation is analogous to the development of red and white muscles in vertebrates and that it serves a similar function, white muscle mainly supporting burst-type swimming and red muscle sustaining steadystate oxidative work.Ofall the Mollusca, squid are by far the most capable swimmers, some being able to attain speeds of 10-15 knots (1, 2). These speeds are attained by cyclic inhalant and exhalant movements of the mantle muscle to produce a characteristic jet propulsion movement. From visual observations of pelagic squid, at least two kinds of swimming (steady-state and burst) can be clearly deciphered. In vertebrates (3), and fish in particular, red, intermediate, and white muscle types are specialized for these two kinds of locomotion, although there may be some overlapping functions in active species like the tuna (4). However, except for a few casual observations (5), such an organization has never been described in cephalopods (6).In reexamining this problem in a number of squid species, we have found clear evidence of metabolic differentiation ofthe mantle musculature. In one common pattern, found in Loligo opalescens, two distinct bands on the inner and outer edge of the mantle are specialized toward a highly oxidative metabolism, whereas the middle mantle displays a higher dependence on anaerobic glycolysis. Evidence from enzyme measurements, histochemistry, and polarized light plus interference contrast microscopy indicates that, compared to the middle mantle, the two oxidative bands ofmuscle display relatively higher activities of oxidative enzymes, higher mitochondrial abundance, and lower myofibrillar density. Variations on this theme were observed in the five squid species examined. While this work was in progress, parallel and independent studies on Alloteuthis (7) showed greater mitochondrial abundance and capillarity in outer and inner bands of the mantle, thus confirming and extending our observations.
285Aspergillus nidulans has at least two permeases for L-proline. The prnB gene of the prn gene cluster specifies the major proline permease, which is inducible by proline. Synthesis of the prnB permease is subject to repression by ammonium at 37 "C but not at 25 "C. A genetically unidentified minor proline permease(s) does not respond to proline induction or ammonium repression but is inhibited by ammonium. areAr mutants are unable to utilize nitrogen sources other than ammonium because they lack a positive-acting regulatory product required for expression of ammonium-repressible activities. However, there are very few cases in which the lack of growth of areAr mutants on a particular nitrogen source can be attributed to a reduction in the level of a particular enzyme activity or permease. Reduced expression of the prnB permease can account for the inability of areAr mutants to utilize proline. This is demonstrated by the ability of cis-acting regulatory mutations designated prnd, which derepress synthesis of the prnB permeasz, to suppress areAr mutations for proline utilization. The apparent ability of prnd mutations to derepress synthesis of proline oxidase and 1 -pyrroline-5-carboxylate dehydrogenase is probably an indirect consequence of their ability to derepress synthesis of the prnB permease, preventing inducer exclusion. There is presently no evidence that prnd mutations directly affect expression of the p m A , prnC or prnD genes, but this possibility has not been definitively eliminated.
Work reported here on the fungus Aspergillus nidulans has provided the first definitive demonstration of operon-type organisation in an eukaryote genome. It has been shown that the prnA and prnB genes concerned with proline metabolism form a gene cluster with the regulatory region lying between the two putative structural genes prnA and prnB. Regulatory mutations (prnd) probably leading to relief of carbon catabolite repression, map in between prnA and prnB and are cis-dominant with respect to both. The properties of these regulatory mutations and other findings suggest that carbon catabolite repression may be mediated by a negative control system in A. nidulans. This gene cluster is particularly interesting in view of its divergent orientation (with the regulatory region located in the centre of the operon) and for the fact that unlike the divergent operons known in prokaryotes, the divergent orientation is related to the way in which this particular operon may be regulated.
Feline immunodeficiency virus (FIV) has a worldwide distribution among feral and domesticated cats and in many cases induces immunodeficiency disease analogous to that of human acquired immune deficiency syndrome. FIV is genetically homologous to human immunodeficiency virus (HIV) in both genome organization and gene sequence and, like HIV, exhibits enormous sequence variation throughout the range of host species. We sampled 91 feral cats from six disparate locales and studied the phylogenetic relationships of viral DNA from infected cats using both pol and env genes (520 and 684 bp, respectively). The patterns from the two genes recapitulated previously described major FIV clades and showed concordance between phylogenetic patterns of the pol and env genes. Evidence for recombination between the pol and env genes was not found among a sampling of nine isolates, although evidence for intragenic exchange within the env gene was observed in two isolates. A small local population of cats from a rural farm in the United Kingdom had a remarkably high FIV antibody prevalence (47%), but displayed 8-fold less overall diversity of FIV genomic variation compared with FIV from different parts of the world. We interpret this low variation as a consequence of a recent monophyletic introduction of FIV into the population.
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