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Duck hepatitis B virus particles bearing the L and S envelope proteins bind a cellular glycoprotein of 180 kDa (gp180) with high affinity and specificity. Binding is mediated by the pre-S region of the L protein and is blocked by neutralizing but not by non-neutralizing monoclonal antibodies to the virus. These and other properties have suggested that gp180 may be a component of the viral entry machinery. Here we report the purification of gp180 from duck liver and the isolation and characterization of cDNA encoding it. DNA sequence analysis of this cDNA indicates that gp180 is a novel member of the basic carboxypeptidase gene family.

The structure of the unknown modified nucleoside Q, which is present in the first position of the anticodons of Escherichia coli tRNA Tyr, tRNA His, tRNA Asn, tRNA Asp, is proposed to be 7-(4,5-cis-dihydroxy-1-cyclopenten-3-ylaminomethyl)-7-deazaguanosine (1). The structure of Q was deduced by means of its uv absorption, mass spectrometry, proton magnetic resonance spectroscopy, and studies of its chemical reactivity. The structure of Q is unique since it is a derivative of 7-deazaguanosine having cyclopentenediol in the side chain at the C-7 position. This is the first example of purine skeleton modification in a nucleoside from tRNA.

The small RNAs of Moloney murine leukemia virus (M-MuLV) were fractionated into at least 15 species by two-dimensional polyacrylamide gel electrophoresis. The pattern of small RNAs is significantly different from that of Rous sarcoma virus. A subset of the virion small RNAs is associated with the genome RNA in the 70S complex. One of the associated molecules, a cellular tRNA, is tightly bound to the genome RNA and serves as the major primer for M-MuLV RNA-directed DNA synthesis in vitro. RNA tumor virus particles such as murine leukemia virus or Rous sarcoma virus (RSV) contain two major size classes of RNA, the high-molecular-weight genome RNA and lowmolecular-weight 4 to 7S RNA (1-3, 7, 8, 12, 19, 21, 25, 27). Some of the low-molecular-weight RNAs are associated with the genome RNA in a 70S complex, whereas others are released free in solution after disruption of virus particles (8, 12, 19, 21, 27). One of the 70S-associated 4S RNAs in RSV virions, the host cell tRNA'r , has been shown to be the primer for initiation of RSV RNA-directed DNA synthesis in vitro (6, 11, 14, 22). To determine whether other viruses use tRNA's as primers, we analyzed the small RNAs of Moloney murine leukemia virus (M-MuLV). One ofthe 70S-associated 4S molecules can serve as the major primer for reverse transcription in vitro. This RNA dissociates from the genome RNA at higher temperatures than do other associated species. An identical molecule is found in uninfected mouse, rat, and chicken cells, suggesting that the M-MuLV primer is a host cell RNA. In a later communication, we will show that this molecule has the amino acid acceptor activity and nucleotide sequence of tRNApro. MATERIALS AND METHODS Virus growth and isolation. The cloned line of M-MuLV designated clone 1 was propagated in a 3T3

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