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Primary mRNA transcripts in several systems are edited by single base substitutions, small deletions or insertions to yield functional messenger RNA species. Mitochondrial mRNAs in particular, including those from plants, seem to be the subject of extensive editing, unlike mRNAs encoded by chloroplast DNA, for which the prediction of amino-acid sequence from the corresponding gene sequence is generally unambiguous. Occasionally, however, an ACG codon appears at the 5' terminus of chloroplast genes, where the initiation codon ATG would be expected. Here we present evidence for a C----U editing that is responsible for the conversion of the ACG codon to an AUG initiation codon in the mRNA transcript from the rpl2 gene of the maize plastome, showing that mRNA editing can also occur in chloroplasts.

The interactions of 4-thiouridine and 5-[(methylamino)methyl]-2-thiouridine in tRNA and of phosphorothioate esters in nucleic acids with an organomercurial have been investigated. For this purpose, an affinity electrophoretic system has been developed in which the mercury derivative has been covalently immobilized in a standard polyacrylamide gel. The retardation of thiolated macromolecules was found to be sensitive to the chemical environment of the sulfur atom, giving characteristic interaction constants dependent on the nature of the modification and its accessibility to binding. The interaction could, in the case of tRNA, be abolished by conventional specific chemical modification of the thiolated bases, as well as by irradiation with 32P-derived beta-emission. Not only has the fractionation of sulfur-modified from unmodified species been attained but a quantitative application of the technique has made it possible to study the binding of mercury and, by competition, that of magnesium in terms of the conformation of tRNA.

The axonal projection mutations irregular chiasm C of Drosophila melanogaster comap and genetically interact with the roughest locus, which is required for programmed cell death in the developing retina. We cloned the genomic region in 3C5 by transposon tagging and identified a single transcription unit that produces a major, spatially and temporally regulated mRNA species of -5.0 kb. Postembryonic expression is strong in the developing optic lobe and in the eye imaginal disc. The gene encodes a transmembrane protein of 764 amino acids with five extracellular immunoglobulin-like domains and similarity to the chicken axonal surface glycoprotein DM-GRASP/SC1/BEN. Both known irreC alleles reduce the level of transcription, whereas the roughest cT mutation disrupts the intracellular domain of the protein.[Key Words: Cell adhesion; Immunoglobulin superfamily; DM-GRASP; optic chiasms; structural brain mutant; verticals]Received August 19, 1993; revised version accepted September 27, 1993.The assembly of a functional nervous system requires the numerical matching and precise connection of neuronal populations, which are often spatially distant. This is achieved through the remarkable ability of developing axons specifically to find and follow the pathways leading to their synaptic targets (for review, see Bixby and Harris 1991;Doherty and Walsh 1992;Hynes and Lander 1992) and through the degeneration of surplus cells (Hollyday and Hamburger 1976;Katz and Lasek 1978). The elucidation of the molecular mechanisms responsible for axonal guidance, neural recognition, and the triggering of cell death is therefore essential to an understanding of the basic developmental strategies generating the intricate pattern of neural organization seen in the adult.In the past few years several studies have provided new insights into the cellular and molecular cues required for correct axonal pathfinding. These include the molecular cloning and characterization of a number of cell surface Present addresses: tlnstituto

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