A comparison was made of the calculated mol. wt. of RNA fragments from foot-and-mouth disease virus (FMDV) types A strain 119, C Resende and O Brugge. The mol. wt. were calculated by two methods: from measurements of strand lengths in the electron micrographs and from the observed sedimentation rates (s-rates). RNA extracted from virus by dialysis against water usually had three to four prominent strands of different lengths. Mol. wt. calculated from s-rates (and converted to strand lengths) generally agreed with those measured on electron micrographs. Differences which occurred appeared to be attributable to further breakage during processing for electron microscopy. Major fragment strand lengths range from 0.65 to 2.45 µm. The modal lengths of RNA fragments were preparation-dependent, ranging from 1.25 to 1.95 µm for A 119, 1.05 to 1.75 µm for C Resende, and 1.65 to 2.45 µm for O Brugge. There was one fragment length 1.95 µm common to all three types of FMDV RNA and several others which appear in at least two types. Calculations using the molar ratios of nucleotide residues in FMDV RNA, a mol. wt. of FMDV RNA of 2.65 × 10 and an internucleotide spacing of 3.17 Å indicate that intact FMDV RNA should be 2.62 µm long and therefore would contain approx. 8270 nucleotides. The derived mathematical expression for the relationship between mol. wt. (M) and s-rate (S) giving the best fit for all data was M = 1725 S, a result close to that derived by Spirin (1963) for other single-stranded RNAs.


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