In the Neurospora genome duplicate sequences are detected and altered in the sexual phase. Both copies of duplicate genes are inactivated at high frequency, whether or not they are linked. Restriction sites change, and affected sequences typically become heavily methylated. To characterize the alterations of the DNA, duplicated sequences were isolated before and after one or more sexual cycles. DNA sequencing and heteroduplex analyses demonstrated that the process (termed RIP) produces exclusively G-C to A-T mutations. Changes occur principally at sites where adenine is 3' of the changed cytosine. A sequence duplicated at a distant site in the genome lost approximately 10 percent of its G-C pairs in one passage through a cross. A closely linked duplication of the same sequence that was passed twice through a cross lost about half of its G-C pairs. The results suggest a mechanism for the RIP process.

We studied the anatomy of neuromasts, afferent sensory neurons, and efferent neurons of the midbody branch of the posterior lateral line in larvae of the zebrafish (Brachydanio rerio), 5 days after fertilization. This simple sensory system consists of ten or 11 neuromasts, 15-20 sensory neurons, and about nine efferent neurons. The neuromasts are typical free neuromasts and both afferent and efferent synapses are present on hair cells within them. The sensory neurons project into a single longitudinal column of neuropil in the hindbrain. The sensory terminals appear by light microscopy to contact the dorsolateral dendrite of the ipsilateral Mauthner cell. Three types of efferent neurons are present; two types in the hindbrain and one type in the diencephalon. We provide several lines of evidence that demonstrate that these central neurons are efferent to the lateral line. We conclude from this morphology that the larval system includes all of the components of the adult system and is probably functional at this early stage. We also found that larvae have all of the efferent neurons found in adult zebrafish, while the number of neuromasts and sensory neurons will increase during subsequent development.

In Escherichia coli, unprotected linear DNA is degraded by exoV activity of the RecBCD nuclease, a protein that plays a central role in the repair of double‐strand breaks. Specific short asymmetric sequences, called chi sites, are hotspots for RecBCD‐promoted recombination and are shown in vitro to attenuate exoV activity. To study RecBCD‐chi site interactions in vivo we used phage lambda's terminase to introduce a site‐specific double‐strand break at lambda's cos site inserted into a plasmid. We show that after terminase has cut cos in vivo, nucleases degrade linearized DNA only from the end that does not have a strong terminase binding site. Linearized cosmid DNA containing chi sites in the proper orientation to the unprotected end is degraded more slowly in rec+ E. coli than is chi‐less DNA. Increased survival of chi‐containing DNA is a result of partial inactivation of exoV activity and is dependent on RecA and SSB proteins. The linearization of chi‐containing DNA molecules leads to RecA‐dependent formation of branched structures which have been proposed as intermediates in the RecBCD pathway of double‐strand break repair.

We have developed a system for analysis of histidine-tagged (His-tagged) retrovirus core (Gag) proteins, assembled in vitro on lipid monolayers consisting of egg phosphatidylcholine (PC) plus the novel lipid DHGN. DHGN was shown to chelate nickel by atomic absorption spectrometry, and DHGN-containing monolayers specifically bound gold conjugates of His-tagged proteins. Using PC + DHGN monolayers, we examined membrane-bound arrays of an N-terminal His-tagged Moloney murine leukemia virus (M-MuLV) capsid (CA) protein, His-MoCA, and in vivo studies suggest that in vitro-derived His-MoCA arrays reflect some of the Gag protein interactions which occur in assembling virus particles. The His-MoCA proteins formed extensive two-dimensional (2D) protein crystals, with reflections out to 9.5 A resolution. The image-analyzed 2D projection of His-MoCA arrays revealed a distinct cage-like network. The asymmetry of the individual building blocks of the network led to the formation of two types of hexamer rings, surrounding protein-free cage holes. These results predict that Gag hexamers constitute a retrovirus core substructure, and that cage hole sizes define an exclusion limit for entry of retrovirus envelope proteins, or other plasma membrane proteins, into virus particles. We believe that the 2D crystallization method will permit the detailed analysis of retroviral Gag proteins and other His-tagged proteins.

In the embryonic zebra fish as early as 40 hr after fertilization, the Mauthner cells (M-cells) initiate an escape response, elicited by tactile-vibrational stimulation. The initial part of this behavior is similar to the acoustic startle reflex seen during the larval stage which begins at 96 hr. The embryonic response is directional and is followed by a series of strong tail flexures which are more pronounced than those during swimming. In the embryo the M-cell fired at the beginning of the response and rarely fired again during subsequent contractions; in our experiments the M-cell did not mediate iterative movements of the tail. The M-cell system is probably involved in evoked hatching behavior, as the tactile response is sufficient to rupture the egg membrane and allow the animal to escape. The M-cell sometimes fired spontaneously, which suggests that it might function also in spontaneous hatching behavior which occurs in the absence of phasic stimulation. At 48 hr the M-cell has morphologically mature synapses on its soma and dendrites, but its cytoplasm is relatively undifferentiated; it has few oriented neurofilaments and no distinct axon hillock. During these stages the extracellular M-spike is longer in duration and smaller in amplitude than at later times when the cell is more mature morphologically. Our data suggest that long-term inhibitory control of the M-cell system begins to function at about the time of hatching. At this time the cell is morphologically mature and is richly supplied with synaptic endings over its soma and dendrites.