The Kinetics of Mechanically Coupled Myosins Exhibit Group Size-Dependent Regimes

Naturally occurring groups of muscle myosin behave differently from individual myosins or small groups commonly assayed in vitro. Here, we investigate the emergence of myosin group behavior with increasing myosin group size. Assuming the number of myosin binding sites (N) is proportional to actin length (L) (N = L/35.5 nm), we resolve in vitro motility of actin propelled by skeletal muscle myosin for L = 0.2–3 μm. Three distinct regimes were found: L < 0.3 μm, sliding arrest; 0.3 μm ≤ L ≤ 1 μm, alternation between arrest and continuous sliding; L > 1 μm, continuous sliding. We theoretically investigated the myosin group kinetics with mechanical coupling via actin. We find rapid actin sliding steps driven by power-stroke cascades supported by postpower-stroke myosins, and phases without actin sliding caused by prepower-stroke myosin buildup. The three regimes are explained: N = 8, rare cascades; N = 15, cascade bursts; N = 35, continuous cascading. Two saddle-node bifurcations occur for increasing N (mono → bi → mono-stability), with steady states corresponding to arrest and continuous cascading. The experimentally measured dependence of actin sliding statistics on L and myosin concentration is correctly predicted.     

Streptomonospora algeriensis sp. nov., a halophilic actinomycete isolated from soil in Algeria

A halophilic actinomycete strain, designated H27^T, was isolated from a soil sample collected from a hypersaline habitat in Djelfa Province (North-Central Algeria), and then investigated using a polyphasic taxonomic approach. The strain was observed to produce poor aerial mycelium, which formed short chains of oval to cylindrical-shaped spores at maturity, and non fragmented substrate mycelium. The optimum NaCl concentration for growth was found to be 10–15 % (w/v) and the optimum growth temperature and pH were found to be 28–37 °C and 6–7, respectively. The diagnostic diamino acid in the cell-wall peptidoglycan was identified as meso-diaminopimelic acid. The predominant menaquinones of strain H27^T were identified as MK-11 (H₄) and MK-10 (H₆). The major fatty acids were found to be iso-C_16:0, anteiso-C_17:0, 10 methyl C_17:0 and 10 methyl C_16:0. The diagnostic phospholipids detected were phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, phosphatidylcholine and phosphatidylinositol. The chemotaxonomic properties of strain H27^T are consistent with those shared by members of the genus Streptomonospora. 16S rRNA gene sequence analysis indicated that strain H27^T is most closely related to Streptomonospora alba DSM 44588^T (98.8 %) and Streptomonospora flavalba DSM 45155^T (98.7 %) whereas the DNA–DNA relatedness values between strain H27^T and the two type strains were 17.1 and 57.9 %, respectively. Based on the combined genotypic and phenotypic evidence, it is proposed that strain H27^T should be classified as representative of a novel species, for which the name Streptomonospora algeriensis sp. nov. is proposed. The type strain is H27^T (=DSM 45604^T =CCUG 63369^T =MTCC 11563^T).     

Mzabimyces algeriensis gen. nov., sp. nov., a halophilic filamentous actinobacterium isolated from a Saharan soil,and proposal of Mzabimycetaceae fam. nov.

Three halophilic mycelium-forming actinobacteria, strains H195^T, H150 and H151, were isolated from a Saharan soil sample collected from Béni-isguen in the Mzab region (Ghardaïa, South of Algeria) and subjected to a polyphasic taxonomic characterisation. These strains were observed to show an aerial mycelium differentiated into coccoid spore chains and fragmented substrate mycelium. Comparative analysis of the 16S rRNA gene sequences revealed that the highest sequence similarities were to Saccharopolyspora qijiaojingensis YIM 91168^T (92.02 % to H195^T). Phylogenetic analyses showed that the strains H195^T, H150 and H151 represent a distinct phylogenetic lineage. The cell-wall hydrolysate was found to contain meso-diaminopimelic acid, and the diagnostic whole-cell sugars were identified as arabinose and galactose. The major cellular fatty acids were identified as iso-C_15:0, iso-C_16:0, iso-C_17:0 and anteiso-C_17:0. The diagnostic phospholipid detected was phosphatidylcholine and MK-9 (H₄) was found to be the predominant menaquinone. The genomic DNA G+C content of strain H195^T was 68.2 mol%. On the basis of its phenotypic features and phylogenetic position, we propose that strain H195^T represents a novel genus and species, Mzabimyces algeriensis gen. nov., sp. nov., within a new family, Mzabimycetaceae fam. nov. The type strain of M. algeriensis is strain H195^T (=DSM 46680^T = MTCC 12101^T).     

Phosphate and ADP Differently Inhibit Coordinated Smooth Muscle Myosin Groups

Actin filaments propelled in vitro by groups of skeletal muscle myosin motors exhibit distinct phases of active sliding or arrest, whose occurrence depends on actin length (L) within a range of up to 1.0 μm. Smooth muscle myosin filaments are exponentially distributed with ≈150 nm average length in vivo—suggesting relevance of the L-dependence of myosin group kinetics. Here, we found L-dependent actin arrest and sliding in in vitro motility assays of smooth muscle myosin. We perturbed individual myosin kinetics with varying, physiological concentrations of phosphate (P_i, release associated with main power stroke) and adenosine diphosphate (ADP, release associated with minor mechanical step). Adenosine triphosphate was kept constant at physiological concentration. Increasing [P_i] lowered the fraction of time for which actin was actively sliding, reflected in reduced average sliding velocity (ν) and motile fraction (f_mot, fraction of time that filaments are moving); increasing [ADP] increased the fraction of time actively sliding and reduced the velocity while sliding, reflected in reduced ν and increased f_mot. We introduced specific P_i and ADP effects on individual myosin kinetics into our recently developed mathematical model of actin propulsion by myosin groups. Simulations matched our experimental observations and described the inhibition of myosin group kinetics. At low [P_i] and [ADP], actin arrest and sliding were reflected by two distinct chemical states of the myosin group. Upon [P_i] increase, the probability of the active state decreased; upon [ADP] increase, the probability of the active state increased, but the active state became increasingly similar to the arrested state.     

Descriptions of digeneans from <Emphasis Type="Italic">Sardina pilchardus</Emphasis> (Walbaum) (Clupeidae) off the Algerian coast of the western Mediterranean,with a complete list of its helminth parasites

Five species of digeneans parasitic in the pilchard Sardina pilchardus (Walbaum), a little studied host, from off the Algerian coast of the western Mediterranean are redescribed. These are Parahemiurus merus (Linton, 1910) Manter, 1940, Aphanurus stossichii (Monticelli, 1891) Looss, 1907, Aphanurus virgula Looss, 1907, Lecithaster confusus Odhner, 1905 and Pronoprymna ventricosa (Rudolphi, 1819) Poche, 1926. One of these, A. virgula, is a new record for this host. One other digenean, Hemiurus luehei Odhner, 1905, was also recorded from this host. A complete checklist of the helminth parasites of S. pilchardus throughout its distributional range, comprising 104 host-parasite records of 39 taxa, is presented.     

Taxonomy and chemical characterization of new antibiotics produced by Saccharothrix SA198 isolated from a Saharan soil

Actinomycete strain SA198, isolated from a Saharan soil sample of Algeria, exhibited antimicrobial activity against Gram-positive and Gram-negative bacteria, and phytopathogenic and toxinogenic fungi. The morphological and chemotaxonomic characteristics of the strain were consistent with those of the genus Saccharothrix. Analysis of the 16S rRNA gene sequence of strain SA198 showed a similarity level ranging between 97.2 and 98.8% within Saccharothrix species, S. australiensis being the most closely related. Two new active products were isolated by reverse HPLC using a C18 column. The ultraviolet–visible (UV–VIS), infrared (IR), mass, and ¹H and ¹⁴C nuclear magnetic resonance (NMR) spectra showed that these products were new bioactive compounds. The minimum inhibitory concentrations of these antibiotics showed a strong activity against fungi and moderate activities against Gram-positive and Gram-negative bacteria.     

Replication slippage may cause parallel evolution in the secondary structures of mitochondrial transfer RNAs

Presence of the dihydrouridine (D) stem in the mitochondrial cysteine tRNA is unusually variable among lepidosaurian reptiles. Phylogenetic and comparative analyses of cysteine tRNA gene sequences identify eight parallel losses of the D-stem, resulting in D-arm replacement loops. Sampling within the monophyletic Acrodonta provides no evidence for reversal. Slipped-strand mispairing of noncontiguous repeated sequences during replication or direct replication slippage can explain repeats observed within cysteine tRNAs that contain a D-arm replacement loop. These two mechanisms involving replication slippage can account for the loss of the cysteine tRNA D-stem in several lepidosaurian lineages, and may represent general mechanisms by which the secondary structures of mitochondrial tRNAs are altered.