pH-Dependent urea sensitivity of southern bean mosaic virus

The behavior of southern bean mosaic virus (SBMV) virions in urea was markedly pH dependent and revealed several novel features. In the absence of urea (5 degrees , 1 hr) SBMV was stable as a 115 S entity between pH 2.5 and 9.5, yielded a slowly sedimenting (ca 60-65 S) entity at pH 10.5, and was degraded into a 35-40 S product at pH 11.0. In 4.0 M urea, SBMV was precipitated irreversibly at pH 2.5 or 10.0, yielded variously sedimenting aggregates at pH 3.0, was transformed progressively into a 80 S entity between pH 7.0 and 9.5, or remained structurally intact at pH 3.5 to 6.5. In 8.O M urea, SBMV was precipitated at pH 3.5 and below or pH 7.5 and above, showed a tendency for the particle:particle interaction at pH 6.5 and 7.0, but remained stable between pH 4.0 and 6.0. Under conditions where SBMV was somewhat destabilized with urea, increase in the temperature or ionic concentration caused virion precipitation without any detectable degradation. SBMV was dissociated with 4.0 or 8.0 M urea at pH 5.5 but only in the presence of 1.0 M NaCl and at the elevated temperatures (>50 degrees ). These results on the rather unusual SBMV behavior in urea, as compared to other viruses, are discussed in relation to the available information on the SBMV stabilizing interactions.