Muscle contraction consists of a cyclical interaction between myosin and actin driven by the concomitant hydrolysis of adenosine triphosphate (ATP). A model for the rigor complex of F actin and the myosin head was obtained by combining the molecular structures of the individual proteins with the low-resolution electron density maps of the complex derived by cryo-electron microscopy and image analysis. The spatial relation between the ATP binding pocket on myosin and the major contact area on actin suggests a working hypothesis for the crossbridge cycle that is consistent with previous independent structural and biochemical studies.

Localization of beta-actin messenger RNA to sites of active actin polymerization modulates cell migration during embryogenesis, differentiation and possibly carcinogenesis. This localization requires the oncofetal protein ZBP1 (Zipcode binding protein 1), which binds to a conserved 54-nucleotide element in the 3'-untranslated region of the beta-actin mRNA known as the 'zipcode'. ZBP1 promotes translocation of the beta-actin transcript to actin-rich protrusions in primary fibroblasts and neurons. It is not known how the ZBP1-RNA complex achieves asymmetric protein sorting by localizing beta-actin mRNA. Here we show that chicken ZBP1 modulates the translation of beta-actin mRNA. ZBP1 associates with the beta-actin transcript in the nucleus and prevents premature translation in the cytoplasm by blocking translation initiation. Translation only occurs when the ZBP1-RNA complex reaches its destination at the periphery of the cell. At the endpoint of mRNA transport, the protein kinase Src promotes translation by phosphorylating a key tyrosine residue in ZBP1 that is required for binding to RNA. These sequential events provide both temporal and spatial control over beta-actin mRNA translation, which is important for cell migration and neurite outgrowth.

Sequence divergence .589 Incidence and level of competence in natural isolates.589 Physiological effects of DNA uptake .590 Environmental Level 590 DEDUCTIVE EVIDENCE FOR BACTERIAL TRANSFORMATION. 590 BIOLOGICAL FUNCTIONS OF DNA UPTAKE OTHER THAN GENE ACQUISITION.591 Regulation of Gene Expression.591 Protection of Cells against Bacteriophages .592 Supply with Nutrients. 592 DNA Repair.592 CONCLUSIONS AND PERSPECTIVES. 592 ACKNOWLEDGMENTS .59 REFERENCES.593 LORENZ AND WACKERNAGEL TABLE 1. Naturally transformable prokaryotic speciesa Species isolated from terrestrial Transformation frequency or aquatic habitats (chromosomal marker Reference(s) transformants/viable cell) Photolithotrophic Agmenellum quadruplicatum Anacystis nidulans Chlorobium limicola Nostoc muscorum Synechocystis sp. strain 6803 Synechocystis sp. strain OL50 Chemolithotrophic Thiobacillus thioparus Thiobacillus sp. strain Y Heterotrophic Achromobacter spp. Acinetobacter calcoaceticus Azotobacter vinelandii Bacillus subtilis Bacillus licheniformis Deinococcus (Micrococcus) radiodurans Lactobacillus lactis Mycobacterium smegmatis Pseudomonas stutzeri (and related species) Rhizobium meliloti Streptomyces spp. Thermoactinomyces vulgaris Thermus thermophilus Thermus flavus Thermus caldophilus Thermus aquaticus Vibrio sp. strain D19 Vibrio sp. strain WJT-1Cd Vibrio parahaemolyticus Methylotrophic Methylobacterium organophilum Archaebacteria Methanobacterium thermoautotrophicum Methanococcus voltae Clinical isolates of pathogenic species Campylobacter jejuni Campylobacter coli Haemophilus influenzae Haemophilus parainfluenzae Helicobacter pylori Moraxella spp. Neisseria gonorrhoeae Neisseria meningitidis Staphylococcus aureus Streptococcus pneumoniae Streptococcus sanguis Streptococcus mutans 4.

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