Synthesis of polyadenylate-containing RNA during the germination of Rhizopus stolonifer sporangiospores

Rhizopus stolonifer sporangiospores synthesized polyadenylate-containing [poly(A+)]RNA de novo during the first 15 min of germination. The length of the poly(A) segments isolated from total RNA gradually decreased from about 100 adenylate residues for spores pulse-labeled during the first 15 min of germination to 60 adenylate residues for spores pulse-labeled for 15 min at 4 h into germination. In contrast, the average length of the newly synthesized poly(A) segments isolated from polysomal RNA[poly(A+)mRNA] was about 50 adenylic acid residues at all pulse periods. The average size of the poly(A+)RNA synthesized during the first 15 min of germination was larger (ca. 1200 nucleotides) than the poly(A+)mRNA (ca. 860 nucleotides). The results are consistent with the hypothesis that the poly(A+)RNA synthesized early in germination is predominantly a precursor of poly(A+)mRNA. As germination proceeds, precursor poly(A+)RNA is processed more rapidly into poly(A+)mRNA and, thus, the majority of the poly(A+)RNA isolated from total RNA at later stages of germination consists of poly(A+)mRNA. If our hypothesis is correct, the processing of precursor poly(A+)mRNA into poly(A+)mRNA involves cleavage of the precursor molecule at both the 5′ and 3′ ends; about 50 adenylic acid residues are removed from the 3′ end and 300 nucleotides are removed from the 5′ end.