nucleocapsid proteins of retroviruses are small basic, nucleic acid-binding proteins with either one or two 'cys-his' boxes, which have been shown to be involved in genomic rna dimerization, encapsidation, and replication primer trna annealing to the initiation site for reverse transcription. the nucleocapsid (nc) protein of moloney murine leukemia virus (momulv ncp10) is made up of 56 residues with one cys-his motif. the zn**2** $plus$ -binding affinities and induced conformational changes of ncp10 were investigated by following the fluorescence of trp 35 located in the cys-his domain. at ph 7.5, ncp10 was shown to bind zn**2** $plus$ at a 1:1 ratio with a very high apparent binding constant of 1.2 ( $plus or minus$ 0.3) $center dot$ 10**1**3m** $minus$ **1. a similar apparent binding constant was obtained for a 19-residue peptide encompassing the cys-his box, designated the 'zinc finger motif,' indicating that it contains most if not all the information to bind zn**2** $plus$ tightly. changing trp 35 to phe in the peptide did not affect the zn**2** $plus$ affinity, indicating that trp 35 is not crucial for zn**2** $plus$ binding. on the contrary, replacing cys 29 by ser, the chemical modification or oxidation of the three cys sharply reduced zn**2** $plus$ affinity, confirming the essential role of cys in zn**2** $plus$ binding. in addition, fluorescence and energy transfer data suggested that zn**2** $plus$ binding modifies the trp 35 environment but not its solvent exposure, and increases the average distance between tyr 28 and trp 35 by about 2 angstrom. these dat