Chickens lack a homolog of mammalian U4A small nuclear RNA.

In mammals, the two major sequence variants of U4 small nuclear RNA, U4A and U4B, differ in primary sequence only at nucleotJde positions 88 and 99 (1). In the chicken, molecular cloning and genomic analysis of U4 RNA genes indicated that there are only two genes In the chicken genome that encode U4 snRNA (2). One of these genes codes for the chicken U4B homotog. However, the second gene, named U4X, does not code for U4A RNA, but rather codes for a more divergent sequence variant that differs from U4B at seven nucleotide positions, although not at nucleotldes 88 or 99 (2). In contrast, an early report from another lab, based upon direct chemical sequencing of chicken U4 snRNAs, suggested that chickens do contain a U4A RNA (3). To resolve this apparent discrepancy, we re-examined the question of whether U4A RNA exists in chickens by using the chain termination method to directly sequence chicken U4 RNAs. The results (Rg. 1) provide no evidence for any sequence heterogeneity In chicken U4 snRNA at positions 88 and 99, whereas two bands are clearly visible in the mammalian case. (The A's at positions 88 and 99 correspond to the U4B sequence, and the G and C to the U4A sequence.) These results, coupled with our molecular analysis of chicken U4 genes (2), indicate that chickens lack a U4A homolog at both the RNA and DNA levels. Thus, at least In the chicken, U4A RNA is not an essential gene product. CHICKEN cONA SEQUENCE MOUSE CDNA SEQUENCE