Bg. Leicht et al., CONSERVATION OF ALTERNATIVE SPLICING AND GENOMIC ORGANIZATION OF THE MYOSIN ALKALI LIGHT-CHAIN (MLC1) GENE AMONG DROSOPHILA SPECIES, Molecular biology and evolution, 10(4), 1993, pp. 769-790
The Mlc1 gene of Drosophila melanogaster encodes two MLC1 isoforms via
developmentally regulated alternative pre-mRNA splicing. In larval mu
scle and tubular and abdominal muscles of adults, all of the six exons
are included in the spliced mRNA, whereas, in the fibrillar indirect
flight muscle of adult, exon 5 is excluded from the mRNA. We show that
this tissue-specific pattern of alternative splicing of the Mlc1 pre-
mRNA is conserved in D. simulans, D. pseudoobscura, and D. virilis. Is
olation and sequencing of the Mlc1 genes from these three other Drosop
hila species have revealed that the overall organization of the genes
is identical and that the genes have maintained a very high level of s
equence identity within the coding region. Pairwise amino acid identit
ies are 94%-99%, and there are no charge changes among the proteins. T
otal nucleotide divergence within the coding region of the four genes
supports the accepted genealogy of these species, but the data indicat
e a significantly higher rate of amino acid replacement in the branch
leading to D. pseudoobscura. A comparison of nucleotide substitutions
in the coding portions of exon 5 and exon 6, which encode the alternat
ive carboxyl termini of the two MLC1 isoforms, suggests that exon 5 is
subject to greater evolutionary constraints than is exon 6. In additi
on to the coding sequences, there is significant sequence conservation
within the 5' and 3' noncoding DNA and two of the introns, including
one that flanks exon 5. These regions are candidates for cis-regulator
y elements. Our results suggest that evolutionary constraints are acti
ng on both the coding and noncoding sequences of the Mlc1 gene to main
tain proper expression and function of the two MLC1 polypeptides.