Molecular and functional analysis of the mariner mutator element Mos1 in Drosophila.

The white-peach allele in Drosophila results from insertion of the transposable element mariner. The particular copy that is inserted in white-peach is an inactive copy referred to as the peach element. The peach element is excised at a high rate in the presence of active copies of mariner located elsewhere in the genome, and the excision of peach in somatic cells is recognized phenotypically by the occurrence of eye-color mosaicism in white-peach flies. Active mariner elements identified by their ability to induce high levels of white-peach mosaicism are denoted Mos (Mosaic) factors. We have sequenced and functionally analyzed the factor Mos1 originally identified in Drosophila mauritiana. The Mos1 element is 1286 base pairs in length, the same length as the peach element. It differs from the peach element in 11 nucleotide positions distributed throughout its length, including four amino acid replacements in the long open reading frame. Analysis of chimeric constructs between Mos1 and peach implies that functionally important differences occur in both the 5' and 3' halves of Mos1. A mariner element identical in sequence to Mos1 yields lower levels of mosaicism in transformants, implying that adjacent flanking sequences have important effects on Mos1 activity. Another mariner element, designated Ma351, isolated from a nonmosaic strain of D. mauritiana, differs from Mos1 in just three nucleotide positions. When introduced into the germline, Ma351 yields various levels of white-peach mosaicism depending on insertion site. These results imply that the activity of mariner elements is determined jointly by their own nucleotide sequences, by the effects of adjacent flanking sequences, and by longer-range position effects.

Molecular genetic analysis of the maize Suppressor-mutator element's epigenetic developmental regulatory mechanism

The genetic mechanism underlying the developmental regulation of the maize Suppressor-mutator element has been analyzed by molecular and genetic techniques. The element is subject to inactivation by a negative, epigenetic mechanism that results in the methylation of C residues in the vicinity of the element's transcription start site. Fully methylated elements are genetically and transcriptionally silent (cryptic), while hypomethylated elements are active. Partially methylated elements, designated programable, exhibit a variety of developmental expression programs. The element encodes a positive regulatory gene product which activates element expression and promotes reprograming of the element by interfering with methylation of the element's 5′ end.Key words: maize transposable element, Suppressor-mutator element, developmental regulation.

The effects of defective Suppressor-mutator insertions on the expression of a Bronze-1 allele in maize

Molecular and biochemical analyses of the maize transposable element mutant bronze-m13, which resulted from the insertion of a defective Suppressor-mutator element in an exon of a Bronze-1 allele, and of changes of state derived from bronze-m13 by internal deletions within the element have revealed how these mutant alleles condition a nonmutant phenotype in the absence of a trans-active Suppressor-mutator. The transposable element insertions are all in the same position, 38 base pairs 3′ to the single intron present in the bz locus. The insertions are transcribed with the gene, and the pre-mRNAs of bronze-m13 and CS1, CS5, CS6, and CS12 are then spliced using the intron donor splice site and either one of two acceptor splice sites in the defective Suppressor-mutator element. Only one of these two messages is translated to yield a functional enzyme. The pre-mRNA of CS9 is spliced only in the reaction that gives a translatable message since the pre-mRNA lacks the alternate acceptor splice site. The splicing reactions are detailed and related to the very different amounts of enzymatic activity produced by these alleles. The presence of an antisense message in CS12 plants to the defective Suppressor-mutator sequence transcribed with the bronze locus is also discussed.Key words: maize transposable element, Suppressor-mutator, splicing.

Is the Suppressor-mutator element controlled by a basic developmental regulatory mechanism?

We report the results of genetic studies on derivatives of two different alleles of the maize a locus with an insertion of the Suppressor-mutator (Spm) transposable element in which the element is inactive, but can be reactivated readily. We present evidence that the mechanism that determines whether the element is in an active or inactive phase has two genetically distinguishable components. One determines whether or not the element is genetically active (the phase setting) and the other determines the stability of the setting in development, its heritability, and its phase in the next generation (the phase program). We show that the element's phase can be reset in a reproducible pattern during plant development. We also show that the Spm element can be reprogrammed to undergo a subsequent phase change without a concomitant phase change. The capacity to reset and reprogram the Spm element is differentially expressed within the plant in a pattern that is correlated with the developmental fate of apical and lateral meristems, suggesting the involvement of a basic developmental determination mechanism.