Periasamy M, Strehler E E, Garfinkel L I, Gubits R M, Ruiz-Opazo N, Nadal-Ginard B
J Biol Chem. 1984 Nov 10;259(21):13595-604.
Fast myosin light chains (MLCf) 1 and 3 are proteins associated with the thick filament in vertebrate fast muscle fibers. MLC1f and MLC3f have complete sequence homology for the first 141 amino acids from their COO- end, but they differ in length and amino acid sequence at their NH+3 ends (MLC1f = 49 amino acids, MLC3f = 8 amino acids), and they are translated from different mRNAs. To elucidate the structural relationship between mammalian fast myosin light chain 1 and 3 isoforms, cDNA clones have been isolated from rat skeletal muscle and their primary nucleotide sequences were determined. MLC1f and MLC3f mRNAs display complete sequence identity in the 3' untranslated sequences (285 base pairs) and in the codons specifying the 142 carboxyl-terminal amino acids. In contrast, the sequences encoding the amino-terminal 49 and 8 amino acids of MLC1f and MLC3f, respectively, as well as their 5' untranslated regions, are highly divergent. Using the cloned cDNAs as probes we have isolated the single gene locus encoding both MLC1f and MLC3f in four overlapping genomic clones spanning over approximately 25 kilobase pairs (kb) of DNA. The sequences encoding the common body of MLC1f and MLC3f are distributed in 4 separate exons at the 3' end of the gene. The start of transcription site and 5' untranslated region of MLC3f is found 5 kb upstream from the common body while the MLC1f mRNA transcription start site and the exon specifying the 5' untranslated region and amino acid 1-40 of MLC1f lies about 10 kb further upstream. Strikingly, a mini-exon that specifies amino acids 41-49 of MLC1f is located downstream from the two MLC3f-specific exons (5' untranslated and amino acids 1-8) and is separated from the upstream MLC1f exons by 12 kb of DNA. This bizarre gene organization implies a novel form of alternative promoter utilization and RNA splicing that is tissue-specific and developmentally regulated in order to generate the mature MLC1f and MLC3f mRNAs from a single gene.
快速肌球蛋白轻链（MLCf）1和3是与脊椎动物快肌纤维粗肌丝相关的蛋白质。MLC1f和MLC3f从其羧基末端起的前141个氨基酸具有完全的序列同源性，但它们在氨基末端的长度和氨基酸序列不同（MLC1f = 49个氨基酸，MLC3f = 8个氨基酸），并且它们由不同的mRNA翻译而来。为了阐明哺乳动物快肌球蛋白轻链1和3同工型之间的结构关系，已从大鼠骨骼肌中分离出cDNA克隆并确定了它们的初级核苷酸序列。MLC1f和MLC3f mRNA在3'非翻译序列（285个碱基对）以及指定142个羧基末端氨基酸的密码子中显示出完全的序列同一性。相反，分别编码MLC1f和MLC3f氨基末端49个和8个氨基酸的序列以及它们的5'非翻译区高度不同。使用克隆的cDNA作为探针，我们在跨越约25千碱基对（kb）DNA的四个重叠基因组克隆中分离出了编码MLC1f和MLC3f的单个基因座。编码MLC1f和MLC3f共同主体的序列分布在基因3'端的4个单独外显子中。MLC3f的转录起始位点和5'非翻译区位于共同主体上游5 kb处，而MLC1f mRNA转录起始位点以及指定MLC1f 5'非翻译区和第1 - 40个氨基酸的外显子则位于再上游约10 kb处。引人注目的是，指定MLC1f第41 - 49个氨基酸的一个小外显子位于两个MLC3f特异性外显子（5'非翻译区和第1 - 8个氨基酸）的下游，并且与上游的MLC1f外显子被12 kb的DNA隔开。这种奇特的基因组织意味着一种新型的可变启动子利用和RNA剪接形式，其在组织特异性和发育调控下，以便从单个基因产生成熟的MLC1f和MLC3f mRNA。
