Heat shock regulatory elements are present in telomeric repeats of Chironomus thummi

As in other Diptera, the telomeres of Chironomus thummi lack canonical shor t telomerase-specified repeats and instead contain complex sequences. They react to heat shock and other stress treatments by forming giant puffs at s ome chromosome termini, which are visible in polytene cells. All telomeres, except the telocentric end of chromosome four (4L), consist of large block s of repeats, 176 bp in length. Three subfamilies of telomeric sequences ha ve been found to show different distribution patterns between chromosome en ds. TsA and TsC are characteristic of telomeres 3R and 4R, respectively, wh ereas TsB is present in the other non-telocentric telomeres. Heat shock tra nscription regulatory elements have been identified in the telomeric sequen ces, appearing differentially represented in the three subfamilies, but oth erwise rather similar in size and sequence. Interestingly, TsA and TsB repe ats share the well-conserved heat shock element (HSE) and GAGA motif, while the TATA box is only present in the former. Neither a HSE nor a TATA box a ppear in TsC repeats. Moreover, experimental data indicate that the HSE is functionally active in binding heat shock transcription factor (HSF). These results provide, for the first time, a molecular basis for the effect of h eat shock on C.thummi telomeres and might also explain the different behavi our they show. A positive correlation between the presence of HSE and telom eric puffing and transcription under heat shock was demonstrated. This was also confirmed in the sibling species Chironomus piger. The significance of heat shock activation of telomeric repeats in relation to telomeric functi on is unknown at present, but it might be compared to the behaviour of othe r non-heat shock protein coding sequences, such as SINE-like and LINE-like retroelements, which have been reported to be activated by stress.