The crystal structure of the mutant S179C of the ribosomal protein L1 from Thermus thermophilus has been determined at 1.9 A resolution. The mutant molecule displays a small but significant opening of the cavity between the two domains. The domain movement seems to be facilitated by the flexibility of at least two conserved glycines. These glycines may be necessary for the larger conformational change needed for an induced fit mechanism upon binding RNA. The domain movement makes a disulfide bridge possible between the incorporated cysteines in two monomers of the mutant L1.