Worm Breeder's Gazette 10(1): 69

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

The Trans-spliced Actin mRNA Leader Sequence is Present on a Myosin Light Chain RNA

C. Cummins and P. Anderson

Krause and Hirsh (1987, Cell: 753-761) have described a 22 
nucleotide leader sequence that is present on mRNA from actin genes 1, 
2, and 3.  This leader sequence is identical to the first 22 
nucleotides of a 100 nucleotide RNA transcribed nearby, but in 
opposite orientation, to the 5S ribosomal gene.  Krause and Hirsh have 
presented evidence that the actin messages acquire the leader sequence 
from the 100 nucleotide RNA by an intermolecular trans-splicing 
mechanism.  The function of the leader sequence remains obscure.
The 'regulatory' myosin light chain (MLC) gene family consists of 
two gene copies, mlc-1 and mlc-2, that are separated by 2.7 kb and 
divergently transcribed.  DNA sequence analysis of the 5' regions of 
the mlc-1 and mlc-2 genes indicated that there is a candidate 3' 
splice site upstream of the ATG initiator codon of each of these genes.
This suggested the possibility that additional untranslated exons 
might be found upstream of one or both of these genes.  Alternatively, 
the mlc messages might acquire a trans-spliced leader sequence.
We prepared synthetic oligonucleotides that are specific for and 
complementary to each of the 'regulatory' MLC transcripts.  To define 
the 5' ends of the mlc-1 and mlc-2 messages, we used these 
oligonucleotides in primer extension sequencing experiments.  We find 
that the mlc-2 message contains at its 5' end the same 22 nucleotide 
This sequence is not present in the 4.8 kb genomic region that we 
have sequenced upstream of the mlc-2 coding sequence.  The mlc-1 
message does not include the leader sequence.  Primer extension 
sequencing indicates that there are two sites of transcription 
initiation for mlc-1 that occur within short, repeated sequences at 
positions -38 and -28 relative to the ATG initiator codon.