Site-directed mutations in the Escherichia coli ssb gene were tested for the ability to complement a chromosomal ssb deletion for viability, and only the ssb W54-->G mutation failed to do so at the pSC101 copy level. Non-aromatic amino acid substitutions for SSB Trp-54 (ssb W54-->L and ssb W54-->S) produced the greatest effects on in vivo protein function including altered marker linkage subsequent to generalized transduction, extreme UV sensitivity, and a lack of ability to support SOS induction. Additionally, the ssb-113 (ssb P176-->S) mutation demonstrated the existence of both uvrA-dependent and uvrA-independent components of SOS induction. Although nucleotide excision repair appeared unaffected by alterations in the SSB protein, the mutational analysis suggests a direct role for SSB in recombinational repair.