SPORE MORPHOLOGY IN THE DICKSONIACEAE. I. THE GENERA CYSTODIUM,THYRSOPTERIS, AND CULCITA

The most mature spores available in herbarium specimens of the dicksoniaceous genera Cystodium, Thyrsopteris, and Culcita were studied by scanning electron microscopy and light microscopy, and representative specimens were analyzed to determine the number of spores produced per sporangium. Thyrsopteris and Culcita feature 64-spored sporangia, but Cystodium is consistently 32-spored. Spores were analyzed both in their native state as found on the specimens and in a perine-free state achieved by treatment with sodium hydroxide or acetolysis mixture. The sodium hydroxide assay demonstrated the presence of a perine in Cystodium, Thyrsopteris, and Culcita subgenus Culcita, but no evidence of a perine so defined was found in Culcita subgenus Calochlaena. Spores of Cystodium feature a nearly psilate exine overlain by a striate inner perine and a granular outer perine and are in several respects similar to those of Metaxya in the cyatheoid-dicksonioid complex and to those of Saccoloma in the dennstaedtioids. The most mature Thyrsopteris spores available may not have been fully mature. They feature a sparsely distributed, granular perinous layer over a microverrucate sculptine. The latter topography is taken as largely perinous since treatment with sodium hydroxide left a nearly psilate exine. The spore morphologies of Cystodium and Thyrsopteris reinforce the taxonomic distinctness of these monotypic genera indicated by their other characters. The two subgenera of Culcita are very dissimilar in their spore morphologies. The exine in subgenus Culcita ranges from psilate to slightly microverrucate proximally and distally, with varying margo development. Spores of subgenus Calochlaena are strongly differentiated from those of subgenus Culcita by their exine of broad spinules which vary in their degree of lateral fusion to each other and in the granular appearance of their distal surfaces. Spore morphology in Culcita strongly supports the argument of those who would raise its subgenera to generic rank.     

A model for the temporal organization of X- and Y-type receptive fields in the primate retina

A model is proposed for the temporal characteristics of X-and Y-type responses of ganglion cells in the primate retina. The main suggestions of the model are: (I) The X-type temporal response is determined primarily by the delay between center and surround contributions. (II) The Y-type response is generated in the inner plexiform layer by a derivativelike operation on the bipolar cell's input, followed by a rectification in the convergence of these inputs onto the Y-ganglion-cell. (III) The derivative-like operation is obtained by recurrent inhibition in the dyad synaptic structure.The X-and Y-type responses predicted by the model, for a variety of stimuli, were examined and compared with available electrophysiological recordings. Finally, certain predictions derived from the model are discussed.     

Activation and inhibition of mitochondrial adenosine triphosphatase by various anions and other agents

The activating or inhibiting actions of a variety of anion species and of oligomycin, aurovertin and Dio-9 on the ATPase of a sonic particle preparation of rat liver mitochondria have been characterized by measurements of the relevantV _max,K _i andK _m values.The normalV _max was increased by a factor near 7 by the anions: dichromate, chromate, pyrophosphate, orthophosphate, orthoarsenate and sulphate. The fully activating concentration varied from about 2 mM for dichromate to 150 mM for sulphate. The increase inV _max was accompanied by a time-dependent decrease in (K _i)ADP, but there was no change in (K _m)ATP. The increase inV _max by the activating anions was abolished by aurovertin; but in presence of oligomycin, the lowV _max was increased by the activating anions by the same factor as theV _max in absence of oligomycin.Certain anions, notably azide, decreasedV _max, but did not affect (K _i)ADP or (K _m)ATP. The decrease inV _max by azide and oligomycin were approximately additive. Even at high concentration, Dio-9 was without detectable effect on the ATPase, but it had a gramicidinlike effect on the intact mitochondria.The specificity of the ATPase for ATP relative to GTP was found to be attributable to the high value of (V _max)ATP compared with (V _max)GTP. The values of (K _m)ATP and (K _m)GTP were virtually the same.Some rationalization of these and other supporting observations is attempted in terms of present knowledge of the constitution of the ATPase complex.     

REVISION OF THE TYPE MATERIAL AND NOMENCLATURE OF MASTODONSAURUS GIGANTEUS (JAEGER) (TEMNOSPONDYLI) FROM THE MIDDLE TRIASSIC OF GERMANY

Abstract: The type material of Mastodonsaurus is revised and its complicated taxonomic history resolved. The genus was erected by Jaeger in 1828 without a species name, which was added subsequently by Holl (1829) who named the type species Mastodonsaurus jaegeri. The large tooth on which Jaeger based his Mastodonsaurus is chosen herein as lectotype of the type species. A smaller second individual, represented by a piece of an occiput, was also named by Jaeger in 1828 as Salamandroides giganteus and, owing to the find of a complete skull, was recognized in 1833 by the same author as belonging to the same species as the Mastodonsaurus. Therefore, Mastodonsaurus giganteus (Jaeger, 1828) is a senior subjective synonym of the type species M. jaegeri Holl, 1829. The precedence of the two generic names was chosen in 1834 by the first reviser, von Alberti, in favour of Salamandroides, but all later authors, including von Alberti himself, followed Jaeger, who decided in 1837 to retain the name Mastodonsaurus. The established usage of Mastodonsaurus is preserved formally herein (nomen protectum). The names Batrachosaurus Fitzinger, 1837, and Labyrinthodon Owen, 1841 are unjustified replacement names of Mastodonsaurus. The names M. jaegeri von Meyer, 1832, S. jaegeri von Alberti, 1834 and M. salamandroides Jaeger, 1837 are junior homonyms and synonyms of M. jaegeri Holl, 1829, and M. giganteus (Jaeger, 1828), respectively. A recent attempt to replace the universally used Capitosauroidea Watson, 1919 by the unused and newly elevated Mastodonsauroidea Lydekker, 1885 is rejected. Two older synonyms of Mastodonsauridae Lydekker, 1885 (nomen protectum) are rejected as unavailable (Labyrinthodontidae von Meyer, 1842 ) and nomen oblitum (Batrachosauridae Fitzinger, 1843), respectively. The holotype of Mastodonsaurus giganteus is reinstated and valid on the basis of three diagnostic features present: the tripartite posterior rim of the parasphenoid, a laterally pushed suture between the parasphenoid and basipterygoid, and a wide slit‐like eustachian tube opening. Mastodonsaurus is known from specimens representing a continuous growth series, now also encompassing the lectotype of Mastodonsaurus jaegeri, which until rather recently stood isolated from other specimens as the largest find by far.     

Is the egg size determining gene,Esd, on the W chromosome identical with the sex-linked giant egg gene,Ge, in the silkworm?

The presence of the egg size-determining (Esd) gene, which acts as a quantitative gene, on the W chromosome of the silkworm was revealed in a previous study by using two types of triploid females, ZZW and ZWW, (Kawamura, Genetica 76: 195–201). The females with the sex-linked giant egg (Ge) gene deposit eggs as large as those laid by tetraploids. If the Ge mutant is induced by translocation of a fragment of the W carrying Esd onto the Z by chance, the egg size increase in the Ge strain and in tetraploids may be easily explained by the double dose of Esd. The measurement of the length of the Z-W bivalent in oocytes showed that the Z of the Ge strain was much longer than that of the other strains which do not carry the Ge gene. The result suggests that the Ge gene is identical with the Esd on the W chromosome of the silkworm.     

Taxa, individuals, clusters and a few other things

The recognition of species proceeds by two fairly distinct phases: (1) the sorting of individuals into groups or basic taxa (‘discovery’) (2) the checking of those taxa as candidates for species-hood (‘justification’). The target here is a rational reconstruction of phase 1, beginning with a discussion of key terms. The transmission of ‘meaning’ is regarded as bimodal: definition states the intension of the term, and diagnosis provides a disjunction of criteria for recognition of its extension. The two are connected by a spectrum, with purely theoretical definition at one pole and purely ‘operational’ diagnosis at the other, with the more operational elements explained by the more theoretical. The current plethora of species concepts provides a good example. Accepting the Ghiselin–Hull thesis, that a species is an individual, a basic taxon is therefore also an individual with organisms as its parts. In a generalised synchronic individual its parts are conceptually integrated by an integrating principle (IP), which consists of a relation applying within a plan or rule. Fully developed, such an IP ensures the maximisation of the information content of the individual. A diachronic individual is then the set of its component synchronic parts, and its IP is provided by near-identity in an appropriate space-time (not necessarily physical). The integration of parts of an individual is illuminated by Gasking's concept of a proper group (in this case a chain-group), whose members are related by the relation serially fitting together with, the IP completed by an appropriate plan or rule. Gasking also applied the term cluster to a proper group persisting over a substantial period of time, individuated by any member acting as focus. A basic taxon is therefore a cluster of individuals, integrated by the relation significantly taxonomically similar and the rule in character-space-time. The nature of those concepts is discussed and defended. A species will inherit certain of the attributes of the preceding basic taxon (taxa). In at least the synchronic version its parts (individuals) will resemble each other significantly, providing the intuitive applicability of ‘members of’ and ‘instances of’ a species. Also, the notion that a species’ name is given by pure ostension, via a name-bearer (holotype) is empirically incoherent: the name cannot be applied in practice without an appropriate set of diagnostic traits.     

WOLBACHIA INFECTION IN DROSOPHILA SIMULANS: DOES THE FEMALE HOST BEAR A PHYSIOLOGICAL COST?

Fitness traits of three Drosophila simulans strains infected by endocellular bacteria belonging to the genus Wolbachia have been compared with those of replicate stocks previously cured from the infection by an antibiotic treatment. The traits measured were development time, egg-to-adult viability, egg hatch, productivity, fecundity, and the number of functional ovarioles. Individuals of the first strain were bi-infected by two Wolbachia variants, wHa and wNo. The second strain was infected by wHa, the third one by wNo. The Wolbachia studied here cause cytoplasmic incompatibility (CI), a high embryonic mortality (70% to > 90%) when an infected male is crossed with an uninfected female. Three generations after antibiotic treatment, we observed in all strains a significant drop in productivity in the cured stocks. This drop was not due to antibiotic toxicity and was associated with the loss of the Wolbachia. However the effect had disappeared in two of the three strains five generations after treatment, and could not be found in the third strain in a third measurement carried out 14 generations after treatment. The temporary nature of the productivity difference indicates that Wolbachia do not enhance productivity in infected strains. On the other hand, in all traits measured, our results show the absence of any negative effects of the Wolbachia on their host. This could be explained when considering Wolbachia evolution, as maternally transmitted parasites bear a strong selective pressure not to harm their female host. However, CI would allow the bacteria to be maintained even when harming the female. The apparent absence of deleterious effects caused by these Wolbachia might result from a trade-off, where a relatively low bacteria density would advantage the Wolbachia by suppressing any deleterious effects on the female host, at the cost of a weaker maternal transmission rate of the infection.     

Molecular cloning and characterization of alc the gene encoding allantoicase of Neurospora crassa

Summary Purrtins can be utilized as a secondary nitrogen source by Neurospora crassa during conditions of nitrogen limitation. The expression of purine catabolic enzymes is governed by the nitrogen regulatory circuit and requires induction by uric acid. The major positive-acting nitrogen regulatory gene, nit-2, turns on the expression of the purine catabolic enzymes, which may also be subject to negative regulation by a second control gene, nmr. We have cloned alc, the structural gene which encodes allantoicase, an inducible enzyme of the purine degradative pathway. The identity of the alc clone was confirmed by restriction fragment length polymorphism analysis and by repeat-induced mutation. The alc gene is transcribed to give a single messenger RNA, approximately 1.2 kb in length. The negative-acting nmr gene affects the expression of alc in the expected manner. Both the nit-2 and the nmr control genes affect alc mRNA levels and allantoicase enzyme activity in both the induced and nitrogen-repressed conditions.     

Some Behavioral Features of Drosophila melanogasterStrains Carrying a flamencoGene Mutation

Olfactory sensitivity and locomotor activity was assayed in Drosophila melanogasterstrains carrying a mutation of the flamencogene, which controls transposition of retrotransposon gypsy. A change in olfactory sensitivity was detected. The reaction to the odor of acetic acid was inverted in flies of the mutator strain (MS), which carried the flammutation and active gypsycopies and were characterized by genetic instability. Flies of the genetically unstable strains displayed a lower locomotor activity. The behavioral changes in MS flies can be explained by the pleiotropic effect of the flammutation or by insertion mutations which arise in behavior genes as a result of genome destabilization by gypsy.     

The distribution in time and space of parasitism in Epinotia tedella (CI.) (Lepidoptera: Tortricidae)

Abstract.

• 1 The temporal and spatial distribution of parasitism in Epinotia tedella was investigated within two spruce stands.
• 2 Most eggs were laid by the host female early in the season. The parasitoid, Pimplopterus dubius, was subjected to a temporal stress, and it also exhibited a greater oviposition rate early in its life. The temporal distribution of attack frequencies by P.dubius and Apanteles tedellae showed a good adaptation to the phenology of the host.
• 3 The spatial distribution of flight activity indicated a preference to exposed sub-habitats by P.dubius and to dark and sheltered sub-habitats by A.tedellae. The distribution of host larvae showed an aggregation in the lower and central parts of the canopy, and so did the attacks by A.tedellae— even more pronounced. The attacks by P.dubius showed a slightly inverse aggregation pattern. Apparently, the distribution of attacks was caused by preference to physical conditions, rather than local host densities.
• 4 The decreasing log. area of discovery on increasing log. parasitoid density is obviously a ‘pseudo-interference’ phenomenon caused by the aggregation of A.tedellae, and temporal asynchrony in P.dubius.
• 5 P.dubius dominated in young stands and its importance decreased with increasing age of the stands. The importance of A.tedellae was greatest in stands between 20 and 40 years old, and it dominated in those stands that were especially dense and dark, due to absent thinnings.

     

Effect of acidicpH on the protein carmin from safflower seed (Carthamus tinctorius)

The effect of a decrease inpH on the structural integrity of carmin has been monitored by a variety of biophysical techniques. The protein undergoes initial dissociation up topH 3.5–4.0 without any significant denaturation. Below thispH the process of dissociation and denaturation appears to be simultaneous. Further, in thepH range of 2.5–1.6 the protein reassociates to probably a different polymer resulting from possibly, an entropically driven hydrophobic interaction. The process of dissociation appears to be reversible to a large extent. The process of denaturation appears to be governed by the kinetic path that the denatured protein molecule follows either by a sudden decrease inpH or through a gradual decrease inpH. These results are interpreted while keeping in view the oligomeric and globular structure of carmin at neutralpH. The results would help in understanding of structure-function relationship of the protein and its role in hydrogen ion bindingin vivo.     

Origin of the main class of repetitive DNA within selected Pennisetum species

In an attempt to identify relationships among genomes of the allotetraploid Pennisetum purpureum Schumach and closely related Pennisetum species with which it can be successfully hybridized, repetitive DNA sequences were examined. Digestion with KpnI revealed two highly repetitive fragments of 140 by and 160 bp. The possibility that these sequences could be used as genome markers was investigated. Average sequences were determined for the 140 by and 160 by KpnI families from P. purpureum and P. squamulatum Fresen. Average sequences (based upon four or five repeats) were determined for the P. glaucum (L.) R. Br. 140 by KpnI family and the diploid P. hohenackeri Hochst. ex Steud. 160 bp KpnI family. The average sequences of the 160 by KpnI families in P. purpureum and P. squamulatum differ by only nine bases. The 140 by KpnI families of the three related species, P. purpureum, P. squamulantum, and P. glaucum are nearly identical, and thus likely represent a recent divergence from a common progenitor or a common genome. Each repetitive sequence may contain internal duplications, which probably diverged following amplification of the original sequence. The 140 by KpnI repeat probably evolved from the 160 by KpnI repeat since the missing 18 by segment is part of the internal duplication that is otherwise conserved in the subrepeats. Tandemly arrayed repetitive sequences in plants are likely to be composed of subrepeats which have been duplicated and amplified.Florida Aqricultural Experiment Station series #R-02758     

Facilitative interactions between two lepidopteran herbivores of Asimina

Summary Insect herbivores that require young foliage for successful larval development are often restricted to a single generation during a year by the scarcity of suitable food over most of the growing season. The major specialist herbivore attacking shrubs in the genus Asimina in Florida, Eurytides marcellus, requires young foliage for successful larval development. Field manipulations were used to investigate the role of the young foliage produced by Asimina in response to defoliation by the late-season feeder Omphalocera munroei, a second specialist herbivore of Asimina in Florida, in maintaining Eurytides populations during the summer months when young foliage is otherwise scarce. Defoliation by Omphalocera proved to be the major inducer of young growth during the summer because Omphalocera defoliated Asimina shrubs so frequently and severely. When compared to young leaves produced in the absence of damage, the teaves produced by Asimina in response to defoliation were equally as suitable as food for Eurytides larvae and as acceptable as oviposition sites by Eurytides females. The availability of young foliage in an Asimina population was correlated with the size of the associated Eurytides population. The combination of regular, severe defoliation by Omphalocera and lack of a defensive response to damage by Asimina lead to a positive affect of Omphalocera on Eurytides population size, and may be central to other facilitative interactions between herbivores as well.     

The two genes encoding protein synthesis initiation factor eIF-5A in Saccharomyces cerevisiae are members of a duplicated gene cluster

Summary Translation initiation factor eIF-5A is an abundant protein in which a lysine residue is modified by spermidine to form the amino acid derivative, hypusine. The factor is encoded by two genes in Saccharomyces cerevisiae, called TIF51A and TIF51B, which are regulated reciprocally by oxygen and by heme. TIF51B, also called ANBI, is located on chromosome X in a region called COR. We physically mapped TIF51A and its associated serine tRNA₂ gene by the method of chromosome fragmentation and pulsed-field gel electrophoresis. TIF5IA maps 90 kb from the left end of chromosome V in a region called ARC. The COR and ARC regions contain CYCI and CYC7, respectively, and appear to be duplications carrying numerous related genes. The arrangements of related genes in the two regions are incompatible with a duplication mechanism involving a circular intermediate.     

Estimation of Fractional Turnover Rate of Aldolase in Skeletal Muscle of the Adult Rat by Simultaneous Administration of Glutamic Acid-1 -14C and Glutamic Acid-3-3H

The spc operon of Escherichia coli encodes 11 ribosomal proteins and SecY. The secY gene and downstream rpmJ encoding a ribosomal protein, L36, are located distal to the promoter of the spc operon. It has been suggested that the stability of SecY mRNA depends on rpmJ unless a ρ-independent terminator is inserted immediately downstream of secY. Moreover, it has been suggested that RpmJ is dispensable for E. coli. We constructed rpmJ null strains, AY101 (ΔrpmJ::tetA) and AY201 (ΔrpmJ::cat), by replacing rpmJ with tetA, which encodes a membrane protein responsible for tetracycline-resistance, and cat, which encodes a cytoplasmic chloramphenicol acetyltransferase, respectively. Depletion of RpmJ did not inhibit protein synthesis, whereas the growth of AY101 was defective at high temperatures. The level of SecY mRNA decreased significantly in both disruptants even though the ρ-independent terminator was inserted immediately downstream of secY. Some periplasmic proteins were missing in the disruptants with a concomitant increase in the amount of phage shock protein in the inner membrane. These phenotypes caused by the rpmJ null mutation were corrected by a plasmid carrying secY, but not by one carrying rpmJ.     

Patch formation by herbivorous fish in a temperate Australian kelp forest

Summary The odacid fish Odax cyanomelas feeds on the kelp Ecklonia radiata, an important component of subtidal reef habitats on the central coast of New South Wales, Australia. Herbivory by Odax has a major impact on the structure and dynamics of discrete patches within larger stands of kelp at Cape Banks. This three-year study showed that each year, between August and October, approximately the same patches of kelp were denuded by preferential feeding on the meristem and primary laminae. This coincided with a variable pulse of Ecklonia recruits to the cleared patches, thereby generating patches of a single age-class of plants. Neighbouring areas of Ecklonia forest, non cleared by Odax, consisted of larger, perennial plants, which exhibited more gradual changes in abundance. The seasonal impact of Odax appeared to be due to a change in the behaviour of female Odax during their spawning period. Observations suggested that females aggregate at traditional sites prior to spawning with territorial males in adjacent areas of kelp forest. An alternative hypothesis, that Odax preferentially attacked stands of one-year old Ecklonia plants, was rejected by a field experiment; the establishment of experimental stands of one-year old plants did not lead to increased damage due to Odax or any change in the use of space by the fish. The generality of this effect of fish herbivory is unknown, but this and other Odax species are widely distributed throughout temperate Australia, where Ecklonia is the dominant laminarian alga. The effects of pulsed herbivory by Odax is contrasted to the more continuous grazing by sea urchins in the same system. The latter herbivore has been shown to maintain areas free of Ecklonia, the long-term effects of herbivory by Odax remain unclear.     

Chorological and ecological phenomena in the differentiation and distribution of the Fagion associations in Bohemia and Moravia (Czechoslovakia)

The 10 Fagion associations bound to the western part of Czechoslovakia are conditioned both ecologically and chorologically in their species differentiation as well as geographical distribution. The division of the alliance into suballiances follows primarily the ecological phenomena. Within the Eu-Fagenion the associations form 3 groups conditioned by ecological factors.The Tilio platyphylli-Fagetum, Tilio cordatae-Fagetum, Melico-Fagetum and Carici pilosae-Fagetum represent associations of the submontane belt. The Tilio platyphylli-Fagetum is characterized by overlapping of Fagion and Carpinion species and by the absence of any Dentaria species due to chorological causes. The Tilio cordatae-Fagetum does not show any chorological phenomena in the species composition, however, it is limited to central, southern and western Bohemia only. The Melico-Fagetum is characterized by dominant Melica uniflora which is absent in the western and southern part of Bohemia. The Melico-Fagetum has even a more limited distribution occurring in northern and eastern Bohemia and in northern and central Moravia only. The Carici pilosae-Fagetum is characterized by Carex pilosa (dominant), Cephalanthera longifolia and Euphorbia amygdaloides; it is confined to the Carpathian province.The Dentario enneaphylli-Fagetum, Dentario glandulosae-Fagetum and Violo reichenbachianae-Fagetum represent a group of vicarious associations of the montane belt. The Dentario enneaphylli-Fagetum occurs mainly in the geographical province eská vysoina. Its eastern limit lies in the westernmost part of the Carpathian province where it forms a special subassociation and contacts the Dentario glandulosae-Fagetum. The latter association is characterized primarily by Dentaria glandulosa, a Carpathian endemic species. The Violo reichenbachianae-Fagetum is conditioned chorologically by the absence of any Dentaria species; it occurs in the mountains Kruné hory and Doupovské hory only.The Festuco-Fagetum is the single representant of the third group conditioned mainly by ecological factors.Nomenclature of species follows rothmaler et al. (1970).     

Photoperiodic Control of Developmental Diapause in Nymphs of Prostriate Ixodid Ticks (Acari: Ixodidae)

Extrinsic control of developmental diapause in nymphs of prostriate ticks of the subgenus Ixodes sensu stricto (Ixodes ricinus and Ixodes persulcatus from Eurasia and Ixodes scapularis from North America) appears to be based on a complex two-step photoperiodic reaction of a short-day/long-day type. Diapause control in the subgenus Afrixodes (the South African tick Ixodes rubicundus) appears to be based on a simple long-day reaction. The option between non-diapause development and diapausing arrest in engorged nymphs is determined by both pre- and post-feeding photoperiodic regimes. Consequently diapausing arrest in engorged nymphs of Ixodes sensu stricto can be induced either by a short-day (after their engorgement) or by a long-day regime (in unfed nymphs), while active, non-diapause development is possible only when the short-day pre-feeding regime is followed by a long-day post-feeding regime. The photoperiodic response in I. (Afrixodes) rubicundus nymphs seems to be of the long-day type both before and after feeding. Consequently this non-diapause development is enabled by a long-day regime, while diapause is induced by a short-day regime of exposure. Nevertheless, there are some indications that the control of nymphal diapause in the latter species is also of a complex nature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Regulation of the Escherichia coli secA Gene Is Mediated by Two Distinct RNA Structural Conformations

Expression from the secA gene, encoding a key component of the general secretory pathway of Escherichia coli, is influenced by the secretion status of the cell, autogenous translational repression, and translational coupling to the upstream gene, X. SecA binds to its mRNA in a region overlapping its ribosome binding site, thus competing with ribosomes that would initiate secA translation. Mapping of the geneX-secA mRNA secondary structure has demonstrated that the RNA can adopt two distinct conformations in solution. The first conformation arises from the base-pairing of the secA Shine-Dalgarno (SD) sequence with the geneX terminus. The second conformation, in which the secA SD sequence is no longer paired with the geneX terminus, contains a GC-rich stem upstream of the secA SD sequence. The presence of this GC-rich stem is supported by structure mapping of a mutant RNA containing a deletion in the geneX terminus. The former structure appears to be involved in translational coupling by directly linking the geneX and secA sequences, where geneX translation activates secA translational initiation through the unpairing and unmasking of the secA SD sequence. As indicated by SecA-RNA binding assays, the latter structure is probably involved in SecA binding and translational repression of the secA gene. The stabilizing effect of magnesium ions toward occlusion of the secA SD sequence supports the presence of RNA tertiary structure in this regulatory domain. Received: 30 July 1998 / Accepted: 18 September 1998